The subtraction operator “-” can be used as a binary or unary operator.
As a binary operator, it subtracts the second argument from the first one.
As a unary operator, it returns the negation of the operand.
Any argument passed as a string is automatically converted into a number.

The following two expressions with a binary operator return the same result.

10 - 4
"10" - "4"

The following two expressions with a unary operator are return the same result.

- 4
- "4"

Boolean values are converted into numbers (0 for FALSE and 1 for TRUE) when they are involved in a subtraction.
Read BLANK handling in DAX for a complete description of the effect of BLANK with different operators.