Shift coverage
Coverage is how badly a shift needs to be filled, which is a different question from how many people it needs. A mandatory shift is one the workplace cannot run without; an optional one is work it would like done if somebody is free.
Why it matters in a small workplace
Every week has more slots in it than people, so the order they are filled in is the whole judgement. A week with every optional slot covered and one mandatory slot empty is not a covered week.
How Weeksort handles it
Weeksort carries three tiers — mandatory, important and optional — and fills them in that order. A mandatory slot still short is flagged in red while you build, and publishing a week that has one asks you to confirm.