By default whatever you set as an employees regular pay will use the system pay code "BASIC".
You may want to change this for some employees, perhaps to differentiate between pay to employees and pay to directors or to analyse pay by department.
Before you can change the code used you need to create a new code that is suitable.
Create a new Pay Code and ensure its set as an 'Addition' code and the calculation method is 'Fixed Amount'.
Now that you have created a suitable code, a new option will appear when you edit the employees pay options.
The option is labeled "Pay Code" and will default to "BASIC".
You can now change update this option and select the new code you just created instead.
Remember that setting the employee defaults only effects future payruns.
If you have a payrun already open then you'll need to edit the regular pay on the payrun to change the code there too.