“Level Off” (v7.1j is the new “Adjust Vertical Speed” (v7.0) which in turn was the new “Reduce Climb / Descent” (v6.04).
These are all weakening of initial corrective RAs.There is some interesting history there, relating to what the call meant versus the original call versus what the aircraft was actually doing, particularly in the case of non compliance with the original RA.
There were also cases of pilots pitching in response to the words of the callout without referring to the (type dependent) vertical speed or pitch targets on their PFD. This can lead to excessive changes of ROC and larger deviations from clearance, increasing the probability of “knock on” encounters with other aircraft.
Note that the 7.0 wording forces you to refer to your PFD. “Adjust my vertical speed? How Much? Better look in now, not out”. This was a major step forward. However, since the intent with all weakening RAs is to level off, at 7.1 it was decided it was clearer to just explicitly say so.
in practice, a corrective RA will generally weaken anyway as either RA is big enough to achieve the target separation. Especially at low level, where that is only 300’. So there is only a very narrow altitude band (150’) where you get inhibited before you would have weakened anyway. In this case the other aircraft just weakens slightly later than it would have.
Incidentally, “Maintain Vertical Speed” is a preventative RA and thus wouldn’t apply in this case. It is used when the flight path (a) needs to stay the same and (b) when the aircraft is already climbing or descending. It can be modified with the word “crossing” if altitude crossing is unavoidable, to try and mitigate the natural human desire not to carry on climbing (or descending) when the intruder is above (or below) you.
When a preventative RA is required in level flight (or gradual climb/descent) this is when you get “Monitor Vertical Speed”.
Ultimately folks, all TCAS RA callouts mean the same thing: look at your PFD and fly just out of the red.
It needs to be done promptly, so in the air don’t overanalyse the words just crack on with it.
Hope that helps.