From FIXwiki

FIX Field 158

The amount the buyer compensates the seller for the portion of the next coupon interest payment the seller has earned but will not receive from the issuer because the issuer will send the next coupon payment to the buyer. Accrued Interest Rate is the annualized Accrued Interest amount divided by the purchase price of the bond.

Notes

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Don't forget to refer to the FPL Discussion Groups to back up anything you write.
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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.