Mark Allen
Programmer- Elite: Dangerous
Frame of reference seems to be fixed until you switch flight modes.
Upon entering a system you are in the main star's frame of reference, but if you leave SC near another body, you enter that object's frame of reference and stay there until you enter SC again.
To clarify, while you're in supercruise you're in the frame of reference of whichever body in the system has the highest gravitational influence on your ship (bar some exceptions that slightly inflate the sphere for tiny bodies that are near massive ones and would otherwise be dominated entirely). In practice which object you're in the frame of doesn't affect too much unless you're travelling very slowly for a long time, as the speeds your ship can achieve dwarf any stellar body... Aside from that one bug I remember when a superluminal moon overtook me :/.
What your maximum speed is limited by is calculated on a similar basis, it just does a little bit of number-fudging to normalise scales - without which large gas giants would take exceptionally long to get away from if you start close.
@OP - yes, you're never stationary in SC! (in fact, in fiction if you're ever not travelling at speed in the direction your ship/FSD is pointed you drop out of cruise - interdiction uses that by trying to pull the opposing ship out of alignment, there's more in the pipe that uses this as well further down the line).