White keys in project mode

What are the white keys are used for when saving a project? So when pressing and holding P the black button of the active project is blinkng. You can save project by pressing and holding the black button.

But what are the white keys are for in this mode? I can press and hold a white key and it seems to do something (the bar runs from left to right), but I don’t know what its purpose is.

If i understand your question correctly, the white keys in this case would allow you to save your chain patterns.
In other words, If you created a chain pattern within your project, you’ll be able to save it by holding the white keys.

If you still have default projects which came form the OP-Z, you’ll see that all of them have at least 1 white key selected (number 1) which stores the chain pattern you’ll play when hitting the play button.

Hope this makes sense