The choice of a greenhouse glazing material (general term for the covering over the greenhouse frame) will depend on how the greenhouse is being used. In general: clear film or covering is a good choice for starting seeds and for quick initial plant growth. A translucent film or other covering is a better choice for long-term greenhouse production. While less light is transmitted to the plants (% light transmission depends on the features of the specific covering material selected), a translucent covering diffuses the light and helps eliminate shadows. It can also help with overheating issues by contributing to reduction of the interior temperatures.
I'm happy to provide more specific advice if needed. There are many other features available on plastic films, rigid and semi-rigid coverings that should be considered as well (anti-condensation, uv stabilizers, etc.) Please share the details as a reply to this message or feel free to email me directly: Brooke.email@example.com