FIX Field 66
Unique identifier for list as assigned by institution, used to associate multiple individual orders. Uniqueness must be guaranteed within a single trading day. Firms which generate multi-day orders should consider embedding a date within the ListID field to assure uniqueness across days.
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Don't forget to refer to the FPL Discussion Groups to back up anything you write. You will often find the authoritative statements on matters of FIX specification interpretation there. Links from FIXwiki pages to relevant discussions on the FPL site are very valuable.
For example if you were writing a clarification on how to specify the "quantity" of an order in FIX, you might add the following relevant link, http://www.fixprotocol.org/discuss/read/f5014573, to back up your points.