Interesting to note that 'floor singer' is a UK only concept? I suppose we don't use the term 'open mic' as most clubs (in my experience, over the last 30 years) don't use any PA.

In my experience, the most common structure is for the 'resident' to open with a couple of songs. Selected floor singers, i.e. volunteers, get a couple of songs each if time permits. Then the main act ('guest') closes the first 'half'. The same structure repeats for the second 'half'.

On a singer's night, there is no guest, and this is the time when a floor singer not known at the club gets the best chance to perform.

But there are variations. One club I used to go to, the Bull at Blackmore, Essex, NEVER had 'guests'.

Similarly, when I was involved, many years ago, with folk clubs in Dubai and Sharjah, there were no professional guests, and we had to rely solely on amateur performers (many of whom were very good). We had almost everything from banjo-pickers to Flemish bagpipes.