Side berths run lengthwise along the corridor, opposite the bays. The side lower converts into two facing seats by day and is the berth two RAC passengers share, each with a seat, until one of them is allotted a full berth. On older ICF coaches it is noticeably shorter than a bay berth; LHB coaches improved it.
On a ticket the code SL after the berth number means side lower; SL as the class on the same ticket means Sleeper. The two are unrelated and the context makes clear which is meant.
Also written or searched as: side lower · side lower berth · side lower RAC