Functionally: a little notification near the top of a profile page indicating the latest time this profile posted to any RP or chat where any of your characters (or possibly just your selected character) are present. Benefits:
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Indicating whether you've messaged a profile before, without having to use contacts to mark the same thing, and without having to go through all your RPs to determine this fact
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Indicating at a glance if a profile has been very inactive with you, again without having to audit all your RPs
Implementationally: select all RPs/chats containing this profile, initialize a variable with a timestamp, iterate over all of RPs, if the timestamp of the latest post by this profile is later than the one stored in the variable and any of your profiles (or just your current one) is also in this RP then update the variable, return the final value of the variable after iteration is complete.
Since this only applies to this specific profile's interactions with your characters, it can't be used for alt-hunting - it is information you can already obtain by a longhand manual means.