Optimal rotation is what you are aiming for. When you cant achieve your optimal rotation most jobs have a priority system that the optimal rotation is based on.

So for example

the optimal is HT-ID-Dis-CT-4th-Phleb-TT-VT-FT-4th repeat this rotation in a real fight would break down into this priority system:

1: heavy thrust
2: Chaos Thrust combo
4: Phleb
5: Full Thrust combo

So if a boss jumps you would look at your buffs and debuffs to see what you need to put back on. For DRG's most of the time its easier than that because if its a short break you can just continue your rotation from where you left off and if its long enough that one of your buffs/DoT's is going to fall before you get back to it you can just start over usually without consequence