Merge pull request #32 from open223/adjust-property-docs

Adjust property explanation

explanation/223_overview.md

@@ -29,10 +29,20 @@ Composition is about what entities make up what other entities. For example, a p
 
 ## Telemetry 
 
-A 223 model does not directly provide telemetric data about the real-time operation or past operation of the building systems. It does provide information about the meaning or context of a given data point and it can link to a source of the data values so that an analytics application can find them. If the building has a BACnet building automation and control system, the model can provide the necessary information for analytics or controls software to learn which BACnet object and property corresponds to the desired piece of information. Data points are represented using Properties. There are multiple types of [Properties](https://explore.open223.info/s223/Property.html), such as control points that can be written to ([ActuatableProperties](https://explore.open223.info/s223/ActuatableProperty.html)), sensor points that are observed ([ObservableProperties](https://explore.open223.info/s223/ObservableProperty.html)), either of which can refer to enumerated ([EnumerableProperty](https://explore.open223.info/s223/EnumerableProperty.html)) or quantified ([QuantifiableProperty](https://explore.open223.info/s223/QuantifiableProperty.html)) values. Properties may also be used for mathematical operations, including those that are common in a building such as control logic. This idea is represented using [FunctionBlocks](https://explore.open223.info/s223/FunctionBlock.html). Properties also have many different characteristics which are described by a vocabulary of [EnumerationKinds](https://explore.open223.info/s223/EnumerationKind.html). 
+A 223 model does not directly provide telemetric data about the real-time operation or past operation of the building systems.
+It does provide information about the meaning or context of a given data point and it can link to a source of the data values so that an analytics application can find them.
+If the building has a BACnet building automation and control system, the model can provide the necessary information for analytics or controls software to learn which BACnet object and BACnet property corresponds to the desired piece of information.
+
+In 223, *Properties* are representations of some quality or characteristic of the building which might be observable, quantifiable, and/or actuatable.
+Properties may have an *external reference* which indicates where the information (i.e., data) corresponding to that property may be found -- this is commonly a BACnet object, but other types of external references are possible.
+There are multiple types of [Properties](https://explore.open223.info/s223/Property.html), such as control points that can be written to ([ActuatableProperties](https://explore.open223.info/s223/ActuatableProperty.html)), sensor points that are observed ([ObservableProperties](https://explore.open223.info/s223/ObservableProperty.html)), either of which can refer to enumerated ([EnumerableProperty](https://explore.open223.info/s223/EnumerableProperty.html)) or quantified ([QuantifiableProperty](https://explore.open223.info/s223/QuantifiableProperty.html)) values. Properties may also be used for mathematical operations, including those that are common in a building such as control logic. This idea is represented using [FunctionBlocks](https://explore.open223.info/s223/FunctionBlock.html). Properties also have many different characteristics which are described by a vocabulary of [EnumerationKinds](https://explore.open223.info/s223/EnumerationKind.html). 
+
+Unlike other ontologies like [SSN/SOSA](https://www.w3.org/TR/vocab-ssn), Properties are *not* associated with the inherent phenomena.
+In 223, Properties are associated with a *single* sensor or actuator.
+Think of 223's Properties as the source of the measurement or value which eventually becomes exposed in the building management system as a "Point".
 
 ## Characteristics
 
 A model can also describe the characteristics of the entities in a building. These characteristics express details about entities that are not otherwise expressed by the type, topology, or composition of the entity. Often, these characteristics are linked to specific instances in a model. For example, a characteristic of a pump would be its rated flow. Another pump in the same 223 model may have a different rated flow, but it will use the same 223 class (i.e. [`s223:Pump`](https://explore.open223.info/s223/Pump.html)). These types of characteristics are modeled using [Properties](https://explore.open223.info/s223/Property.html). 
 
-Properties also have various characteristics including their units, quantity kinds, enumeration kinds, and aspects. [Units](https://www.qudt.org/doc/DOC_VOCAB-UNITS.html) describe the unit (e.g. Fahrenheit) of a [QuantifiableProperty](https://explore.open223.info/s223/QuantifiableProperty.html) and [quantity kinds](https://www.qudt.org/doc/DOC_VOCAB-QUANTITY-KINDS.html) describe the type of quantity that may be stated by means of units (e.g. temperature). These are modeled using [qudt ontologies](https://qudt.org/). [EnumerableProperties](https://explore.open223.info/s223/EnumerableProperty.html) do not have units, but they have enumerated values that can be described by [EnumerationKinds](https://explore.open223.info/s223/EnumerationKind.html). Aspects establish the context of a Property. For example, if a Property has a Temperature value of 80.6, aspects are used to state what that represents, such as a Temperature limit during working hours, etc. A Property can have any number of aspects, as needed to establish the context. Any [EnumerationKind](https://explore.open223.info/s223/EnumerationKind.html) can be used as an aspect.
+Properties also have various characteristics including their units, quantity kinds, enumeration kinds, and aspects. [Units](https://www.qudt.org/doc/DOC_VOCAB-UNITS.html) describe the unit (e.g. Fahrenheit) of a [QuantifiableProperty](https://explore.open223.info/s223/QuantifiableProperty.html) and [quantity kinds](https://www.qudt.org/doc/DOC_VOCAB-QUANTITY-KINDS.html) describe the type of quantity that may be stated by means of units (e.g. temperature). These are modeled using [qudt ontologies](https://qudt.org/). [EnumerableProperties](https://explore.open223.info/s223/EnumerableProperty.html) do not have units, but they have enumerated values that can be described by [EnumerationKinds](https://explore.open223.info/s223/EnumerationKind.html). Aspects establish the context of a Property. For example, if a Property has a Temperature value of 80.6, aspects are used to state what that represents, such as a Temperature limit during working hours, etc. A Property can have any number of aspects, as needed to establish the context. Any [EnumerationKind](https://explore.open223.info/s223/EnumerationKind.html) can be used as an aspect.