A drawing number, or any name — "front axle concept" works. Unique within your catalog.
A full http:// or https:// address, or leave it blank.
Node color comes from status and node shape from type; these two are the per-item channels (spec §2).
"Remove link" takes the item out of this assembly and nothing more. The item stays in the catalog — if that was its last use, the "Unused only" filter on the item list is where it turns up.
Not in the catalog yet? Create a new item and add it
Removing an item from one assembly without deleting it is a tree-view action.