The Reply key signalization for partylines in Eclipse ECS/ EHX systems allows a operator to reply back to PL using the reply key . 
The “Prevent reply key signalization” setting works similar to the same setting on directs or interfaces and is present since EHX v7.0. 
It only has an effect if “In-use tally” is ticked for the same partyline (for the directs it is “audio presence tally”) 

So if “prevent reply key signalization” is set to FALSE and “in-use tally” is set to TRUE, and your panel has a Talk  & Listen key for the PL, when the PL goes in-use (another panel or interface talks to the PL)  - the talk key(s) to the PL will start to flash (bright, 4Hz, for however long you have set this for your panel) & the label for the PL shows up on your reply key in the same way a panel that is talking to you directly would.
Reasons for this feature are:

a)     You have a highlyvisible indication letting you know where audio is coming from, good for brightwork environments or stressful situations where multiple sources talk at once
b)     It flashes for anadditional 10 seconds (default setting) after the transmission has finished,good for those short messages where you didn’t have a chance to see who wastalking to you because you have been intensely staring at a screen or mixingdesk
c)      Should the PL thatyou are listening to be on some shift page you are not currently on, you cannow have some visibility and a chance to reply without having to swap back tothat shift page.