Yes, calendar year plans starting January 1st will always expire December 31st, independantly of that setting, which will only influence the new year of expiry after renewal depending on current expiry date and on date of renewal.

4 examples with the setting to allow gaps but not overlaps that i gave you:

1. current subscription expires 2015-12-31. Renewed now, will expire 2016-12-31.

2. current subscription expired 2014-12-31. Renewed now, will expire 2015-12-31 (if bonus time <3months)

3. current subscription expired 2014-12-31. Renewed now, will expire 2016-12-31 (if bonus time >=3months)

4. current subscription expires 2016-12-31. Renewed now, will expire 2017-12-31.

CBSubs manual also explains that setting in details starting page 325 (search for " gap" with space before in the PDF to find it quickly).

Hope that clarifies and corresponds to what I understood from your explanations.