A booking is a named block of hours at one practice site, agreed before the day and confirmed in writing to the person who keeps the rota. The practice sets the start and the finish, the site, the block length, and the split between work booked ahead and work that arrives the same day.
Those four things are settled before a shift is confirmed, so that the rota can be built against them and reception knows what it is booking into. A shift confirmed without them is a shift somebody has to renegotiate on the morning it starts.
Cover is arranged for the shift in front of it. Where a practice wants the same weekday held across a run of weeks, each shift is still confirmed separately with its own hours and its own block length, because demand at a practice moves and a shift booked in the abstract is usually the one that goes wrong.
On the day the locum works to the shift the practice has set. Any change to the block length, the workload or the finish time is agreed with the practice before it is made, and it is written onto the confirmation, so both sides hold the same account of what was worked.
Where a practice prefers to hold a run of dates under one reference, the office will do that and still send a separate line for each date, because a reference is an administrative convenience and a date is the thing that gets worked.