A vapour control layer limits water-vapour movement through a building assembly. Its required characteristics and position depend on the complete construction and conditions. The term does not simply mean any sheet that appears waterproof.
How this term relates to your project
For a roof or wall, materials, junctions and penetrations need assessment together. Adding one layer without understanding the existing build-up may create an unsuitable combination. Ask what function the proposed layer serves and how it connects to adjoining components. Discuss moisture and airtightness separately: those functions may overlap, but are not automatically identical.
Follow the selected system and project assessment when developing details. Retain product information and photographs of important connections before finishes conceal them. These records help establish which layer was actually installed and where penetrations or transitions occur when future investigation or alterations become necessary.