I find it weird that HL2 runners use RTA as a segment attribute.
In my head, the opposite of Singe Segment is not RTA, but Multi Segment. SS is when you do not load in the run, MS is when you load a save.
Same goes for Segmented: the opposite is Non-segmented. Segmented is for runs in which the runner is allowed to retry a segment by loading a save, and not lose time in the process. And this way, one can grid for an optimal segment. Non-segmented does not allow for the runner to retry segments this way.
This way, a SS and MS run can both be RTA, since one can be running against real time in both cases.
To be more precise, the opposite of RTA (in my head) is not IGT, but "using IGT to time a run". Because (in my head), RTA means "using real time to time a run". But to be more brief, I just use RTA and IGT instead.