You only have 2 days out of any global pass you can use in your home country. You can use these on whichever days during the overall validity you want.

As the first post says you are limited to using your pass on up to 2 travel days in your home country. If you have a mobile pass you can use these 2 days however you want, That means you can fly out and train back twice (I did it in 2022). The pass just treats any travel in the UK as valid along with the remaining countries on those 2 days.

You can use unlimited trains on these travel days ( remember UK time is 1 hour behind CET so you must board your last train with a scheduled departure time of 2259 UK time.)

I consider them Country of Residence days, rather than In/Out days, which is how paper passes used to be.