pairId and idx values are used by position-management methods.
maxWithdrawable is the largest collateral removable from the position — the smallest of the three limits the contract checks: the leverage cap (removing collateral raises leverage), liquidation safety (what remains must cover accrued fees and any unrealised loss), and profit protection. It is price-dependent, so it is only current as of the read that produced it.
rolloverSnapshot is the pair’s rollover accumulator when the position opened. With Pair.rollover from getPairs(), it lets you advance accrued rollover locally between polls — see rolloverFee().