Opening and closing shifts
An opening shift is the first shift of the trading day and a closing shift is the last. Both usually need somebody who can unlock, cash up or lock the premises, which is a smaller group of people than the whole team.
Why it matters in a small workplace
A workplace can be fully staffed and still unable to open, because the one person who can unlock is away. That gap is invisible on a grid that counts heads rather than skills.
How Weeksort handles it
Make key holding a qualification and require it on the opening and closing positions. Anybody who does not hold it stops being eligible, so the hole appears while you are building the week rather than at the door.