The phrase "during CPI calculation" should be the key to understanding here. CPI tells you how much prices have gone up or down generally, by just sending a bunch of secret-shoppers around to measure a bunch of prices.
Knowing the treasury's current bond interest rates, the current exchange rate, and Core CPI, you can derive economy-wide aggregate demand—which is the point of the whole exercise. You can then look at any given good's economy-wide demand (i.e. the same formula, where Core CPI is calculated in terms of just that one good) relative to total demand as a time series, take their co-variance, and that's the good's demand-elasticity. (All the numbers are public at every step; there's no real way to fudge them.)
> Would tampons be covered by UBFS? Let's say then we allow for UBFS to cover female menstrual products. Would cups then be covered? Why or why not?
Again: you plug in the demand-elasticity formula and get an answer. No politics, just cold automated logic. Female menstrual products as a category are very likely to be calculated to be inelastic.
Now, how that breaks down to individual items within the category is something I didn't define. As a raw guess, the government could define base inelastic-value for the category, which would apply as a sort of rebate to all the items in that category. Any individual item's price should, after this "rebate", reflect exactly and only its additional elastic demand—making the "premium" products cost some number of nicety-dollars, while the "economy" products in that class cost "nothing" in nicety-dollar terms. (The real key there is the creation of a law preventing companies from increasing their prices in response to this—because, without market manipulation, of course prices would rise to reflect all the same competitive demand as before, plus the extra "free" votes each person got. The law would just need to force companies receiving necessity-dollars to price according to demand in an alternate world where demand was exactly [the amount of inelastic-demand] lower.)
> one word: Arbitrage.
No reason to allow individuals to trade in necessities at all, which would nearly completely prevent arbitraging opportunities. (e.g. it's obvious enough that you shouldn't be able to rent your necessity-dollar-paid apartment out on AirBnB.) Consider anything paid for in necessity-dollars to be tainted and have no legal nicety-dollar value after that.
CPI tells you how much prices have changed, it doesn't explain the source of the changes (and, particularly relevantly, it doesn't measure isolate effects of changes in the supply of dollars chasing the good -- that would be attributed to the degree of elasticity -- vs. other influences on the price of an item.)
there are so many reasons why this wouldn't work. The whole point of UBI is that you are not creating a judgemental system by which certain products are or are not reified by the system. Would tampons be covered by UBFS? Let's say then we allow for UBFS to cover female menstrual products. Would cups then be covered? Why or why not? How would you validate a product as being "for a given use"? How much political maneuvering would be required to get a permit for a product?
* inelastic demand (as found by the BLS during CPI calculation)
Pray tell how does one measure demand elasticity[0]? And if you're entrusting the BLS to do this, will they apply fudge factors? Why or why not? Who gets to influence the BLS to get the 'stats right' on their
* You can't tax or fine anyone in necessity-dollars
one word: Arbitrage.
[0] usually a merchant does this by dicking around with the price.