Specification short code/ identifier

It seems common when building a set of IDS rules to want provide an identifier or reference for the specification for ease / clarity of identification. e.g. a BEP / EIR might have a table similar to

| Reference | Rule Name |
| --- | --- |
| **01.01** | a project Name must be defined|
| **01.02** | the project Name must match 'Acme Project123'|
| **02.01** | a site Name must be defined|
| **02.02** | the site Name must match 'Client site'|
| ... | |
|**19.16** | all doors must have a FireRating property in Pset_DoorCommon with a value being one of '30,60'|
...

In each case the numeric Reference is a handy short code for the spec - where the name may be cumbersome to read/scan, or even duplicated. i.e. Rule **19.16** is easier to talk about than _all doors must have a FireRating property in Pset_DoorCommon with a value being one of '30,60'_. Especially in we change the rule name slightly in future (30,60,90).

Equally Rule **01.02** is more clearly disambiguated from **02.02**.

Currently in IDS1.0 a [specification](https://github.com/buildingSMART/IDS/blob/b12e39224c35dac92f9adff068eb0cba74134621/Schema/ids.xsd#L212) has the following attributes:

* Name (required)
* Description (optional but presumably for longer narratives)
* Instruction (optional for assisting the author)
* Identifier (optional but intended as a machine readable identifier)

Question is where would we put this _reference_ today? Possible options include:

1. In the Identifier - where it's not strictly machine readable / GUID
2. As part of the Name - where it's purely conventional and hard to work with
3. In the Description - which rules out using that attribute for it's purpose.
4. In the Name field - and move the narrative to description - which may cause other UX issues

None feel ideal. I'd like to propose an addition for IDS1.1 of an optional **reference** which is user-facing 'short code' which could be presented alongside the Name, and used for things like sorting, filtering etc in any user interface.