Begin with how the room is used
Casement windows project into or away from the room. Sliding windows remain within the frame plane. Furniture, screens and cleaning access can quickly narrow the choice.
Review ventilation and sealing
Casement sashes commonly use compression seals, while sliders depend on tracks, interlocks and drainage. Neither format should be selected by appearance alone.
Coordinate the elevation
Opening direction, fixed panes and repeated dimensions should be resolved in the window schedule before shop drawings.