From FIXwiki

FIX Field 6

Calculated average price of all fills on this order.

For Fixed Income trades AvgPx is always expressed as percent-of-par, regardless of the PriceType (423) of LastPx (31). I.e., AvgPx will contain an average of percent-of-par values (see LastParPx (669)) for issues traded in Yield, Spread or Discount.

Notes

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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.