You want a calcium with no phosphorous and no D3 to use at all feedings but two a month and on those two (two weeks apart) you want to use a vitamin powder with D3 and prEformed vitamin A in it.
D3 is needed for the chameleon to absorb the calcium out of its diet. D3 can also come from exposure to a proper UVB source such as the direct sun (when it does not pass through glass or plastic) or a long linear UVB bulb such as the Reptisun 5.0 or Arcadia 6% long linear bulb. However, D3 from supplements can build up in the system and lead to health issues but as a rule, D3 produced from exposure to the UVB is converted by the body as needed and should not build up as long as the chameleon can move in and out of the UVB light at will.
Vitamin A comes in two forms…prOformed (beta carotene) and prEformed (retinol) sources. Although the beta carotene won’t build up in the system, it’s thought that chameleons can’t convert the Beta carotene into vitamin A effectively…or maybe not even at all…so we use a prEformed source twice a month to ensure it gets some vitamin A without over doing it. The prEformed sources can build up in the system leading to health issues which is why we only use it twice a month carefully.
In addition to this, D3 and vitamin A are antagonistic to each other and need to be in balance.
Calcium of course is needed for strong bones and muscles and some other things as well…so we want to make sure they get enough.
We eliminate the phos because there is already too much in many of the insects we use to feed the chameleons.
Hope this helps to explain it all a little better.