The Identifier is a name you assign to distinguish a specific sequence
(list) from any others. For example, if you use Word's Insert | Reference |
Caption feature to insert figure and table captions, you can see (by
pressing Alt+F9) that the number is actually a SEQ field: { SEQ Figure \*
ARABIC } or { SEQ Table \* ARABIC }. In these example, "Figure" and "Table"
are the identifiers. You can use any word you want as an identifier, and you
can have as many separate sequences (runs of numbering) as you have
identifiers.

It is just a name label to identify that particular sequence eg {SEQ
Mylist}.