Safety Relay and Contactor Abbreviations

I have some safty control relays like this http://literature.rockwellautomation.com/idc/groups/literature/documents/sg/440r-sg001_-en-p.pdf

and some safety contactors like this http://literature.rockwellautomation.com/idc/groups/literature/documents/pp/100s-pp004_-en-p.pdf

We use Abbreviations on our prints like CR=Control Relay and AH=Alarm Horn,etc.

What should I use some safety relays and safety contactors when both are in the same panel?

SR=Safety Relay and SC=Safety Contactor?

Other Ideas?

Traditionally I think it's K and U, Z or A. I use SC and SU (Safety Unit). It doesn't really adhere to any standard though.