What is a "--mountable" option in rails plugin generator?

rubyonrails-talk
1

There is no any info about "--mountable" option in Rails Guides about plugins. What is this for? Where is it possible to see full explanation?

For now I know that:

rails plugin new my_plugin

and

rails plugin new my_plugin --mountable

create absolutely different directory structures.

2

http://edgeguides.rubyonrails.org/engines.html