Signal TVS vs Power TVS?
You're right that this does not appear to be a standard term. From looking at the differences between TVS diodes on Digi-Key with and without it, though, I think it has a fairly simple meaning. First, it only applies to the steering configuration; you're using zener diodes, so you don't have to care about it at all.

Within the steering configuration, there are two approaches: you can connect the upper diode to your power rail, like on the left:







simulate this circuit &ndash; Schematic created using CircuitLab

This has the disadvantage that any positive ESD strikes get shunted into the power rail, which can raise it to a voltage above its rating. Therefore, the configuration on the right is often used, with a zener diode instead of (or in addition to) the connection to the power rail, thus protecting the power rail from overvoltage: "Power Line Protection".

(If you're wondering why someone would use this latter configuration rather than only a zener diode, it allows several signals to be protected with mostly regular diodes: only one zener.)