BBC explainer:
'If the first minister resigns, the Scottish Parliament has 28 days to choose a new one.
It will be up to the members of the Scottish Parliament (MSPs) to nominate a candidate to replace Humza Yousaf.
By convention, this will be the leader of the party with the most representation in parliament, which is currently the Scottish National Party (SNP).
Nominees are given the opportunity to argue their case and then MSPs cast their vote for a preferred candidate.
When a party has a majority in parliament, this process is a formality - but that is not the case at the moment.
If MSPs can decide on a new first minister, they are then recommended to the monarch for appointment, who rubberstamps the process.
The new first minister then picks their new ministers and leads the government.
However, if the MSPs can't decide on a new first minister, parliament must be dissolved and a new election called.'