STLC is part of SDLC. It can be said that STLC is a subset of the SDLC set.
• STLC is limited to the testing phase where quality of software or 
product ensures. 
SDLC has vast and vital role in complete
development of a software or product.
• However, STLC is a very important phase of SDLC and 
the final product or the software cannot be released without
passing through the STLC process.
• STLC is also a part of the post-release/ update cycle, 
the maintenance phase of SDLC where known defects get fixed or a
new functionality is added to the software.