how is a coastal plain different from a interior plain?

It's essentially the geographic and geologic origin of the plain.

Coastal plains are found from a sea coast and extend inland until they hit another major landform (like a mountain range, etc.). They were originally coastal and continental shelf deposits that were uplifted, or exposed when the sea receded. They can be many miles wide.

Interior plains do not have a boundary that touches a sea or ocean. Some or all of their origins may have been an ancient sea, however. Much of the surface sediments of the North American interior plain were eroded from the Rockies, for example.

These differences affect climate, and therefore natural vegetation.

cp lower elevation, sandy soil, variable winds, salt tolerant plants

ip higher and varied elevation, various soil, westerly winds, wildflowers and grasses