A Qualified Vendor List (QVL) is a list supplied by a system or motherboard manufacturer of parts tested and confirmed by that manufacturer to work in their hardware. This allows them to guarantee specific parts will work in their system.
Most memory sellers, including Crucial, only sell parts compliant with manufacturing standards set by the Joint Electron Device Engineering Council (JEDEC). This means that even if not included on a QVL, similar parts will comply with JEDEC standards and work even if not tested by the hardware manufacturer prior to a system's release.
A QVL not updated after release can also exclude newer technologies allowing higher capacity or faster upgrades than what the QVL or other system specifications may indicate. Also, system updates (BIOS updates or UEFI updates) may be supplied by the system manufacturer itself to enable compatibility with more upgrades, while the QVL may not be updated to reflect that.
Servers and workstation-class systems using ECC memory have additional considerations for compatibility, such as ranking, which a QVL or service manual may disclose. Also see this article for more details.