You can assign values to variables in Service In or Pipeline Out using 
on the Pipeline view toolbar. When you assign a value to a variable, you can:
on the Pipeline view toolbar. When you assign a value to a variable, you can: on the Pipeline view toolbar. When you assign a value to a variable, you can:
Explicitly “hard code” a specific value in a variable.Initialize a set of input variables by assigning values to all of the input variables.Assign a default value to a variable. That is, a value that is only assigned if the variable is null at run time.Assign a variable the value of another pipeline variable by referencing the variable. You might do this if you wanted to derive the default variable value from another variable in the pipeline at run time.Assign a variable the value of a global variable. You might do this for values that will change after you deploy a solution such as connection credentials. Instead of changing the value assigned to the pipeline variable in each service that establishes a connection, you change the value once in the global variable definition. For more information about global variables, see Assigning Global Variables to Pipeline
Variables to assign a value to a variable, you instruct Integration Server to write a specific value to that variable at run time. This action occurs just before the selected service is executed (if you assign the value to a variable in Service In) or immediately after the selected service is executed (if you assign the value to a variable in Pipeline Out).