For float variables... The number specifies how many decimal places the output is written as. For example, if Z is 0.123456, then:
outputs 0.12.filewritevar(Z,2)
For integer variables... The number specifies an alignment. The output is padded to this size, with spaces on the left. For example, if A to 47, then:
outputs " 47".filewritevar(A,5)