The description of the Tag attribute is no longer relevant

The Tag attribute has this description:

> The tag (or label) identifier at the particular instance of a product, e.g. the serial number, or the position number. It is the identifier at the occurrence level.

This does not seem correct. As described by @TLiebich here: https://forums.buildingsmart.org/t/how-do-you-store-element-codes-in-ifc/884/4?u=moult - the two primary places to name things are at the `type.Name` for the type, and the occurrence's `Name` attribute for the occurence's name. This description of "It is the identifier at the occurrence level." is misleading.

Similarly, the examples given, such as serial number should instead be at Pset_ManufacturerOccurrence.SerialNumber, and the other instance, i.e. an actual occurrence's physical tag is at Pset_ConstructionOccurance.TagNumber. This leaves the `Tag` attribute as a rather "vague" occurrence identifier.

To make it less vague, I recommend that the Tag attribute description be changed to reflect its defacto usage:

> If the IFC is a secondary artifact produced via an exporting system, the Tag number may store the identifier of the entity in that originating system.

Locate the IFC4.x specification source for the Tag attribute and review its current description alongside the linked buildingSMART forum discussion. Done means the description reflects the proposed originating-system identifier usage and no longer presents serial numbers or physical tags as Tag examples.

Written by the indexing model from the issue text.