What those this mean?

Question

The Designated Amount shall be defined as increments of either: (a) $50 USD for residents of the United States (b) £25 for residents of the United Kingdom (c) $50 CAD for residents of Canada, or (d) $50 AUD for residents of Australia.

Answer 1

Increments of these amounts mean there won't be any amount not evenly divisible by those amounts.

So the designated amount will be some number of increments of the amount, depending on the currency used.  in Canada for instance, the designated amount will be some multiple of $50 in Candian Currency.   So $250 CAD would be a possible amount, but $247 would not be, because one of the increments was less than $50