After receiving the acceptance letter and paper revised suggestion, you still have an opportunity proofreading your paper and making necessary revision. That is, it is the last chance to modify your manuscript. After the deadline of final submission, your manuscript can no longer be changed. The deadline for uploading your final version of manuscripts is announced in the websites. 

Anyway, if authors do not submit their final version of manuscripts, the original submission of manuscripts will be used for the conference proceedings. That is, the announced final submission deadline is the last date to update or revise the manuscripts. Thereafter, the conference proceedings will be finalized.