One opening.
Several possible scopes.

A sticking panel can involve rollers, track damage, alignment or worn seals. Ask which condition was observed and whether compatible parts are available. A sound frame may remain in place for a targeted repair; deterioration at the sill or frame can change that scope. Compare the actual findings before choosing a complete replacement.

Moisture between the panes is different from condensation on a room-facing surface. A proposal may replace the sealed glass unit while retaining a suitable sash and frame. Ask which component has failed, how the replacement glass is specified and whether the existing frame can support the work.

An insert retains a suitable existing frame and places a replacement unit within it. Full-frame work removes more of the assembly and exposes the opening for assessment. Compare retained materials, flashing work, trim restoration and the resulting clear opening rather than treating both proposals as the same job.

Compare the ratings for the complete product and its suitability for the local climate and project. U-factor and solar heat gain coefficient describe different aspects of performance. Ask which specified unit the label describes; a generic brochure does not establish the rating of the product being supplied.

Describe the opening, how it moves and what changed. Note whether a concern is with glass, the moving panel, hardware or the surrounding wall. If available, gather product labels, earlier invoices and photographs taken without dismantling the assembly. Explain access restrictions and keep people away from broken glass.

Ask which units and components are included, what remains in place, how access and removal are handled, and who restores affected finishes. Confirm product specifications, disposal, weatherproofing and completion checks. Any concealed damage discovered during the work needs an explained change to the agreed scope.