What is the difference between a greenhouse and a tunnel in Heirloom?

Whether it moves: getting it right shows where each covered crop belongs. A Greenhouse is a permanent structure with its own beds. A tunnel, a Caterpillar Tunnel, Mini Tunnel or High Tunnel, sits over some beds of a field block and can move from season to season.

Plantings planned for either one show that cover on the Crop Map, so you know where they go.

See Add a greenhouse and Add tunnels to your field blocks.