The Assign block is used to assign a value to a variable. This assignment is absolute (ex:Y number of gold coins, in my story, I lose all my gold coins and I get back a purse of 10 gold coins. So I add a block Assign whose variable is Y and I add 10 in value by double clicking in the block. The mathematical operation is now Y=10).

The assigned value will correspond to the nature of the variable. So a boolean variable can only have a true or false value, a numerical variable, a numerical expression, ...