This is a subtle distinction.
If you move up to 80 paces you have reached the TZ, and are constrained by it.
Beyond there you have entered the TZ, and are constrained by it.
If you start your move withing 80 paces you are starting in it, and are constrained by it.
If you start your move at 80 paces you are at the edge of the TZ, but are not TZ'ed - you have not reached, entered or started in it.
[Correction - this was the situation in DBMM v1. In DBMM v2 you are in the threat zone if you are at 80 paces]