The number of . (periods) a category has, signifies where your account sits with regards to site-wide permissions and access.
. Can be viewed by all visitors, without a need to be logged in.
.. Can be accessed and contributed to by registered accounts only.
... For those with moderator status.
.... Is for those with web-administrator access.
..... Is for those with permissions and ability to contribute or alter domain controls and content from the front end.
...... Well that's just classified
There may also be sub-categories within each main category that are titled with the . (period) as well, Sub categories simply means categories inside a main category.
Sub categories titled with a . (period) are much the same as defined above, but the permissions and controls granted to your account will only pertain to the content of the main category that they are home to.
For example: Registered members that have followed due process and achieved the user group level that permits moderating any content submitted by a newly registered member may be able to see the main categories . and .. as well as any other sub categories that have more defined titles, but not see sub categories ... , .... , or higher, of the main category . and may not see any of the sub categories out side of sub category . of the main category that is titled ..