Basic Retention Rules for Data/Compliance Archiver Data
Essentially this option is only relevant if you are sending archive jobs (therefore archive data) to the storage policy, if you are not I would suggest you simply set it to the same as Basic.
As has already been said, when/if you start to archive data it's a good idea to use a different storage policy, you don't however have to. You can send both backup and archive data to the same storage policy, so this simply allows you to set different retention periods for backup data and archive data should you so wish.
The extended retention rules allow you to specify a deifferent retention period for specific conditions, consider the following:-
A site performs full backups every Monday-Friday, they have 5 tapes labelled Monday to Friday, these will be overwritten every week so basic retention would be say, 6 days / 2 Cycles, so data will only age after 6 days providing there has been 2 good full backups during that period, a setting of zero cycles would mean the tapes will ALWAYS be overwritten after 6 days, so you could end up in a situation where backups are failing and tapes have all been partially overwritten, so you have no good copies of data, a setting of 6 cycles would mean there would need to be 6 good copies which could never happen as there is only 5 tapes, so data wouldn't age.
Let's say they decide to get 12 additional tapes labelled Jan/Feb/Mar/.....Nov/Dec. these are put in on the first Monday of Every Month, Extended retention rules allow you to specify that these tapes should have a retention of 364 days, without affecting the standard Mon-Fri retention.
In order to get familiar with CommVaults retention rules, I used a test setup to play with storage policy retention and ran reports to look at the state of the media, once you see the effects of the changes you make it really helps you to understand the rules.