Usually re-balancing makes sure everything in the portfolio is at the percentage you chose when you set it up. You can always change your percentages, which I do, after assessing their performance about once a year (changing the allocations). I have the yearly auto-re-balance, but you can do it any time.
Allocations are different than re-balancing. When you decide what percent to put in what category, that is allocation. Re-balancing is moving the money around to be sure everything stays close to the allocations you chose based on your assessment of risk, etc.
If you look at your last statement from TIAA-CREF, you might be able to see what was earning big then, which would be then scaled back to other categories to match the % you assigned it. For example, if Equity Index R1 showed profit that took it above your 18% allotment, then some of it would be moved to, say, your Real Estate category, because it lost money and needed to be brought back up to 8% of the total. If you hover over each category while looking at your account, it will open details about gains for that category. (I know this because I use TIAA-CREF).