1. Generally speaking, a letter represents a specific quantity, which can be variable: for example, Y=X+ 1. When the quantity of X changes, Y also changes, but in the general quantitative relationship, it only represents one quantity. If this quantity is constant, for example, 5=X+ 1, because there is no change on the left (or the result at the left is known, only the unknown quantity needs to be found), so the unknown quantity only has a specific value, but it still represents a quantity. It's just that there is no unnecessary change in this quantity.

2. In general (given) problems, specify a letter to represent something (but not others), such as π for pi, V for volume, n for natural number, etc. This is to facilitate communication.

So strictly speaking, letters in mathematical problems do not need to be classified according to their meanings. Letters are conditions set for convenience of expression.