The Plot journal is where you create whole plots and or sub-plots to be used (or not) later on when you're ready to write but can't think of a good idea.

A writer's mind can work that way: always full of great ideas and when pen goes to paper all you get is a blank...

So, now instead of letting all those good ideas fly through your head and out into oblivion, why not write them down for later use before you have a chance to say "I'll work on that idea later."

Write plot ideas from beginning to end, including conflicts, resolutions, themes, and alternate endings.

 If you find it easier, create story webs to accompany your ideas so that when you go back to it it may make more sense to you.