TPT scripts

Hi ,
I see in the TPT scripts that te syntax of APPLY TO OPERATOR or FROM OPERATOR is APPLY TO OPERATOR (operator_name[1]) and FROM OPERATOR (operator_name[2]).
Can anyone tell me what does this number inside [ ] indicate ?
And also how do we get to know which operator to be given in APPLY TO OPERATOR and FROM OPERATOR.
 
Thank you,
Devika Rani