Third-Party Intermediary (or Intermediaries)
TPI stands for a lot of different things. I'm just giving one of the definitions here, which is used in certain military/ government/ police situations, also in the worlds of business/ finance.
The setting for my example is a hostage situation (if you've watched the first episode of "Flashpoint" you'll recognize this).... One lone hostage taker has a gun to the head of his hostage. Police presence is all around. A civilian who clearly knows the hostage taker is seeking to intervene.
Police 1 to Police 2: "Hold fire, we have a possible TPI here."