I don't quite understand the questions, but I'll attempt an answer. You have all the standard Retirement Account withdrawal options starting on your 55th birthday. Let's assume your Retirement Account is funded to the Full Retirement Sum. One immediate option you have, if you wish, is to make a property pledge or charge (if you don't have one already) and withdraw RA funds, dropping your Retirement Account down as low as the Basic Retirement Sum if you wish.
If you don't withdraw funds and simply start CPF LIFE payouts -- which you can do as early as age 65, as late as age 70, or anywhere in between -- then that's your participation level, for life. (Unless you add funds. You can go up from there if you wish.)
You aren't able to "choose the BRS" and then keep funds above the BRS in your Retirement Account if that's what you're asking. No, you don't get to set aside $X in your RA, above the BRS, to stay in your RA as if it were a piggybank. The very act of "choosing the BRS" involves withdrawing funds.
Note that there could be withdrawal restrictions below the Full Retirement Sum if the source of some of your Retirement Account funds is from Special or Retirement Account top ups, plus accrued interest on the top ups.
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