Question: What is a planning category?
Answer: When you plan in advance for the way your budget is going to be spent, you initially plan for how it's going to be spent over time, that's the most natural thing to do.
So you plan for $1000 in Jan. $1000 in Feb., $3000 in March, etc. You can basically do the same thing with a list of values in lieu of the months.
Say you have a list like this one:
< 1 month
< 6 months
< 1 year
< 3 years
< 10 years
> 10 years
The list is meant to represent when the books acquired where published. Then you plan for it, saying: we went to spend at list 40% of our budgets on books published less than a year ago, 10% on books more than 10 years old, etc.
Upon acquiring new material, you'll be able to select, for a given item, a value from this list in a drop down. Then, after the material has been acquired, at the end of the year, you'll be able to compare the goals set, with what's been achieved.