Strainer type, strainer mesh/perforation size, fluid service, body material, end connections, design temperature/pressure, flow rate are typical

Does it need PI/dPT ports? Dimensional constraints? Solids content?

There's a lot of things you COULD put in the specification. I'd recommend you ask a senior process engineer or at a minimum check out some generic specs from manufacturers to get an idea of what options/features are available.

Also do you want quick open closures ( to ASME VIII normally) or bolted closures.

Also max differential pressure you want clean to dirty ( usually 0.1 to 1 bar)

Drain points and valves or not
Vent point or valves

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

When submitting a PID you are supposedly at the very end of a designing process, where all technical decisions have been made down to the smallest details.

If the project (as very often seen at PID description stage) has not reached this level, or descriptions are not made by engineers qualified as specialists you have two options: get hold of a specialist to make the decisions, or open up verbally by describing verbally exact the effect and targets you want for the process, operation, maintenance and lifetime of the product.

You have already by others got good advice on necessary details, when correct type of filter form and mesh details/types have been selected.

Thanks every one for the great support