JR East-South Hokkaido Rail Pass - flex day

Hi,

I'm interested to purchase and use the JR East-South Hokkaido Rail Pass to travel between Tokyo and Hakodate / Sapporo and parts of Tokyo outskirts like Karuizawa.

I realise there's possibility to choose the pass to be used for 6 days within a 14-day period. Do I need to choose those 6 days upon purchase or those 6 individual dates get "stamped" whenever I use the pass on a new day ?

http://www.jreasthokkaido.com/easthokkaido/e/

Thanks,
dsim

Stamped on days of use.

to be used for 6 days within a 14-day period.

Unsure if you'd be traveling more than a fortnight, just want to share my experience:
I have used this pass twice; as it is valid for 14 days, I have taken advantage of utilizing it for traveling long distances only. Any other day, when we were traveling locally or short distances, we just purchased single tickets. I found this pass could have better value than National JR Pass if you are traveling within the designated areas only.