The travel days on flexible passes, i.e. those with X days within Y months, can be chosen freely within the validity period. 10 days in 2 months means you can use the travel days on 10 dates that you choose within those 2 months.

There are also consecutive passes: 15 days, 1 month, 2 months and 3 months. These passes can be used on all days in the chosen period.

Thanks so much for your response. Just another quick question re. The flexible pass: so I have to book those days that I wish to travel when I actually purchase the ticket? Or can I just show up to a railway station and hop on (using one of my travel days)?

You don't have to choose the travel days in advance. You only have to activate a travel day before boarding your first train on that day.

You can use as many trains as you can find or manage on a chosen day-and as above, can leave actually the choosing till you hit the station. So if you really like a town, can stay longer and re-arrange all those pre-set plans. You can even use an overnight train that starts <24.00 till the end-though nearly all such trains also require an extra REServation.

depending on where/when you travel however, you may have to prebook/Reserve seats/space on fast trains-this varies a lot per country. This hence may restrict your freedom, but its mostly not really needed to panick about them and do it many monthes advance