Most likely for films within 30sec - 1 minute of each other, grouping them together, erring on the side of over-development.
I've done similar in my own personal photography: when I have a bunch of different films, seeing which have similar development times that I can group in my 3 reel Paterson tank.
It's not that much related to film speed, you need to compare the times.
Depending on what volume of film you process, this individuation can add significantly to your cost by requiring more batches than you would otherwise need, which is why sometimes B&W development has a higher charge than C-41, even though the chemicals are generally cheaper.