Confusion regarding bid_unit vs. charge_by



I’m referring to this endpoint:
POST accounts/:account_id/line_items

I’ve read the descriptions for these fields several times and I’m struggling to understand what the functions of these fields are, how they differ, and how to properly use them together.

What’s clear:
These fields only come into play if the objective of the line item is APP_INSTALL

What’s not clear:
In bid_unit:

“Defaults to bid unit based on objective.”

-so this means that the default bid unit is APP_INSTALL?

In charge_by:

" Defaults to charging by bid unit except for line items with bid unit APP_INSTALLS, where it defaults to charging by app-clicks"

-there is a default charge_by value, which insinuates that we can theoretically bid on one action and be charged on entirely different actions? how does this make sense? i’m not really sure how/why these would be used with differing types for bid_unit and charge_by…
-when bid_type=APP_INSTALL, charge_by defaults to APP_CLICK? but in this case we can still change the charge_by value to something else entirely different, right?

“Example Values: APP_INSTALL, APP_CLICK”

-we can define multiple comma-delimited values for this? it doesn’t look like we can from the sample response in which this field is not returned as an array