Hi kev7,
STSL in Payroll Powered By KeyPay is a consolidation of all loans provided by the government to people undertaking higher education, trade apprenticeships and other training programs, such as HELP/SSL/TSL/SFSS.
To find out more about how STSL is calculated in the pay run, see this article here - https://support.yourpayroll.com.au/hc/en-au/articles/360000328095-How-is-STSL-calculated-in-the-pay-...
For more information about STSL and what it encompasses I would recommend visiting the ATO's own website here - https://www.ato.gov.au/individuals/study-and-training-support-loans/
I hope I have pointed you in the right direction, feel free to reach out to us again if you needed any further assistance.
Kristian.