I've been mentioning that for a while now. With these aprons the days of spending a lot of money all the time are over, not just because of the tax, but because of the aprons. And specifically the cost involved in undoing a wrong decision with overvalued, overpaid players that gets you stuck in one of the aprons, specifically the 2nd apron (just look at the Suns as an example, or even Philly before our trade - like they still had to give up two first to get rid of PG13).
When you look at our cap position over the next 4 seasons:
(click to enlarge)


This is probably the most unfun part of the entire NBA for a lot of people. Because a) it's hard to understand it all and b) who wants to figure out accounting when we're watching sports, dreaming about championships? But unfortunately this is the "constraints" part of the game, because the league has a soft salary cap and a set of increasingly punitive punishments if thresholds are exceeded. The first penalty is a financial one (tax) the next two hit you in roster building, by removing your flexibility and (as Brad said, to general derision) "optionality" (I assume he means he wants to have options going forward rather than be locked into a roster and be unable to make changes if plans change.
Since it can be confusing to look at the actual cap, I summarized it to see how much committed money the Celtics have to players for the next 4 seasons, compared to the cap, the tax, the 1st apron and the 2nd apron. (The difference between Spotrac's Active Cap and my "Committed Money" is that I just added Mitchell Robinson and Mike Conley to their Active Cap number, which they excluded because they probably haven't completed signing their contracts yet).
(click to enlarge)

So basically this season we are just over the tax (though with PG13 waiving his trade bonus that might bring us just below it). In any case we have little flexibility now unless we want to exceed the tax again. Since we went below it last season we didn't pay the repeater rate, if we drop below it this season we reset the rate, back to the base rate which is an approximate 300% (it's a proportional tax like income tax so I'm just providing a general rate, don't shoot me if you know the exact bracket rates, I do too but it would be too complicated and boring to explain it here, literally like doing a tax return). If we don't, we go back to the repeater rate which is around 575%.
We reset it if we can go below 2 out of the last 4 seasons. If you look at the table, next season we are already over it with the committed salaries we have with 11 players (assuming PG13 opts in to the last year of his contract). Now they might be able to convince him not to opt in if they offer him a new contract of 3y/$25m or something, but right now we're on the hook for his $56m. And that's not taking into account extensions we have to negotiate with Garza, Banton, Walsh and maybe Conley. So whichever way you look at it, next year is a
big spend year - it's probably not a tax ducking year. Then the year after that we have more players coming off the books that either need to be extended or not, but because we figure to be under the cap with 8 signed players we can sign some free agents before extending our own.
So whenBrad talks about "optionality" that's what I think he's talking about - the flexibility to sign players and make roster changes heading into those big spend years, where you have committed money already where you are going to be over the tax, and maybe the first apron, no matter what. So it's not just enough to say "Tatum has a 10 year prime we have to maximize the whole 10 years", it's picking when of those 10 years you can really make a big impact.
Sometimes I regret ever learning about the cap and aprons and all this stuff (I have people here I can blame for educating me

) because it's not exactly a fun way to dream about championships. We want to dream about championships and great players every season, not think about "spending windows". It does help me better understand why they do things they do though.