FIX support for pairs trading

Imported from previous forum

[ original email was from Roseate L. Wagner - roseate@exchange.ml.com ]
Hi,

We are trying to find out a way to offer the clients to specify an order with a limit relative to an index or some other stock price as for pairs trading through FIX.
Are there any tags or user defined tags established to support such trading strategy?

Many Thanks,
Roseate