From FIXwiki

ExecInst Value 6

An order that may participate in a transaction initiated by another party, but may not initiate a transaction. For example, on US ECNs / Exchanges, this may represent an order that will be quoted to the marketplace and will trade if another party initiates a trade (i.e. hits the posted quote), but cannot be routed to initiate a trade with another market or market maker.

Notes

Error in FIX Specification:

The Glossary (Spec volume 1) refers to this field as "Participate Don't Initiate", Volume 6 refers to it as "Participant don't initiate". It should be consistent. Also, other values of this field use "do not" instead of "don't". Therefore, it is suggested that the name "Participate do not initiate" should be used everywhere for this value.