Rockwell Automation Logix5000 Controllers Add-on Instructions Programming Manual - Manual (Page 20)

Chapter 1 Designing Add-On Instructions
Once generated, the instruction signature seals the Add-On Instruction,
preventing it from being edited while the signature is in place.
In addition, when a sealed safety Add-On Instruction is downloaded for the first
time, a SIL 3 safety instruction signature is automatically generated. The safety
instruction signature is an ID number that identifies the execution characteristics
of the safety Add-On Instruction.
Signature history
The Signature history provides a record of signatures for future reference. A
signature history entry consists of the name of the user, the instruction signature,
the timestamp value, and a user-defined description. Up to six history entries can
be stored. If a seventh entry is made, the oldest entry is automatically deleted.
20 Rockwell Automation Publication 1756-PM010F-EN-P - October 2014