A plan that lasts 12 months but expired at a fixed date whatever is the date of subscription. I have understood in the documentation that "Start of the year" would do the job if I stipulate start of the year on 1 october 2016, then all subscriptions would expire on 30 september 2017.

Correct, that'll work by using the by calendar year plan duration.

What happen if I want to give some tolerance say 2 months. Shall I give a 14 month period ?

Yes, if they subscribe 2 months before the year ends it'll give those 2 months free and subscribe them for next year.

Can I create a subscription plan with a price depending on cb_field options such as Magazine "yes"/"No" ? The cb_field being showed in the subscriber CB profile and the value in € of the "yes" choice being different depending on the plan?

Yes, you can dynamically change a plans price using a CBSubs Promotion. A negative promotion increases the price while a positive decreases it. This can be conditioned based off a field, etc.. Fields can be shown/hidden based off subscription as well. Not sure what you mean regarding the value condition.

Note we have a demo for all our products available below (frontend and backend access is provided) so please do feel free to try this out.

demo.cbdemosites.com/