Certainly at our place, "spare" turns are used to cover longer-term things such as annual leave, safety briefs, vacant lines, longer term sickness and, staff released for other things. They won't pay overtime to cover a spare turn - if something needs covering, they'll use overtime or whatever to cover that directly.
"Service recovery" turns are used to cover stuff that goes wrong on the day - short notice sickness, traincrew displaced by disruption etc.. Subtly different.
so the 'spare' is a float / relief to cover the planned / semi planned absence
and 'service recovery' covers the emergency / short notice
I'm still not sure why it's such an important issue in this discussion though?
the need to have both 'spare' and 'service recovery' staff was questioned
the reality is any workplace shouldn;t have a staffing model that means they only have a relationship with enough staff to cover a normal day's work ...
the employer knows how many hours of annual leave ( with wriggle room) they have to give in each week , they also know roughly how many hours they want to give for training / briefs etc that take people away from the 'shop floor' whether this is on a weekly / monthly/ quarter / full year basis ...
they have a good idea of how much facility time is needed - as the meetings and training stuff is pre -planned and the actual nitty gritty of rep work should be reasonably predictable
employers know roughly what their sick % figure should be / what they are aiming for and can work that into the cover - and as you said once it;s apparent someoen is going to be offfor a period of time or 'long term' you can cover their rota / line witha temporary postign into that line...
short notice unavilability is the un preduictable thing and sometimes this will meand one of two things
1. staff rotaed to cover this unpredictable absence / service recovery etc will sit on their bum in the messroom / control room for most / all the shift
or
2. services will go uncovered as they are un staffed and /or recovery from disruption will take longer as there are no extra staff and trains to fill gaps where trains and/or staff are i nthe wrogn place / out of drivign hours etc etc ...
it's somethign which is not unique to the railway and in some organisatiosn there are complex models of how to use extra hours for 'service recovery ' / major incident issues ( including who not to call as not to dig a bigger hole in 12-18 - 24 hours time )