For the benefit of those who don't have a good understanding of substrates and their properties, I'll begin with the basics. A substrate is any smooth surface over which the flooring product is installed. Substrates may be any of various underlayment types, existing resilient floors, Portland cement concrete, poured gypsum underlayment, and wood panels.
Improper or incorrectly installed substrates can lead to a number of job-related problems. In the case of vinyl floor installations, such problems include tunneling over underlayment joints, wood show-through, discoloration from staining elements in the wood, joint telegraphing, and more.