24K front / 20K back / 14K side: This is the purity that kundan meena jewellery has been made in, traditionally. In this case, the purity of back is 20K while the walls (also referenced as sides) are made in 14K pure gold whereas the gold surrounding polkies is pure 24K.
24K front / 18K hallmark: With standardisation, the concept of hallmark has come in nowadays, where both the back and side are 18K and the front is still 24K pure. One might feel that we should go for hallmark only, but essentially it doesn't make much of a difference. Think of it this way: a hallmarked piece takes more gold to make and hence costs more. Polki is the same in both; so is the look, resale value and everything else.
24K front / 22K hallmark: Both the back and side are 22K pure while the front is 24K pure.
18K hallmark: Open setting style of jadau jewellery essentially involves setting the polkies without use of pure 24K gold that is a prominent feature in regular jadau jewellery. In layman terms, wherever there is gold - it will be 18K pure in such pieces.
24K front / 23K back: Thappa jewellery uses a gold foil of 23K purity on the back and has 24K i.e. pure gold on the front, just like jadau jewellery.
Broadly speaking, there are 3 types of polki: Syndicate, Zimbabwe & Kilwas.
While syndicate and zimbabwe are considered the best and lustrous ones, kilwas is a close second.