It's not crazily complicated but it's not linear also - the community managers look at overall authorized end strength as manadated by Congress, and then take into account promotions, attrition, HYT gates for each year group of Sailors within their community. As everything is cyclical, certain windows may be open (for example, a rate may accept conversions for E-4 and E-5 only because there is a need in certain year groups) in cases or otherwise if the year group is right at allowed end strength then no conversions are permitted. At its most basic level, it's supply vs demand...the 'demand' is set by Congress as the military's allowed end strength (how many total people on active duty), and the supply is regulated by the community personnel managers through recruiting and conversions.
As related to promotions, this is where advancement quotas come from - the same process. The Navy advancement cycles for E-6 and below are Jan - Jun and Jul - Dec so advancement quotas are established for each; depending on the year groups of folks competing during that cycle will affect how many quotas are available.
Same thing for schools - class start every week in San Antonio, with the exception of the weeks around Christmas/New Year and unforseen circumstances; however those seats may be filled up to a year out. Recruiters make reservations for boot camp/schools in their system when getting applicants - it doesn't mean it's guarenteed. Also upon arrival, being on hold may just mean that the school is waiting for everyone to arrive before that particular class starts, especially if people are coming from all over the Navy. Every school is slightly different however.
Best overall answer is that if you are trying to join, do your research ahead of time, know what you want. The recruiter is there to get you in the Navy, not a specific job. Ultimately it's your decision on what to do, but understand that the military as a whole is downsizing right now which greatly affects all manning.
Semper Gumby! Semper Liquidis! Semper Abeo!