Welcome to the forum.
As a rule, the greater the number with the x the thinner the diameter. The pound test varies from brand to brand. Once you get above 0x, you would have sizes larger, x1 or bigger When the X comes first than the larger number with it the heavier it is, but typically the leader or tippet is just reffered to by its pound test.
7x or smaller- leader shy trout
3x-6x- usual trout and panfish
0x-2x- normal bass sized
x1 or larger- heavy cover bass and larger fish
Hope this helps