I understand that Cache is available in the hub and it needs to be emptied once in a while for free credits (à la Saints Row: The Third and the Assassin's Creed series), but how does it work? What do the markings on the Cache checkout mean and how do the total earnings vary? How many extra credits per Cache cash out do I earn for completing other activities? What can I do to increase the total yield of my Cache outside of activities? If I don't empty the Cache when it's full, do I lose any additional benefits I would have earned over that time? How often does it fill up?
That's... a lot of questions. Probably should have a few of those separated off into their own questions. Also, the term 'Cache' is used for two different things, so I'll see what I can do to separate things.
Cache (the resource) is basically what it sounds like; cash. It's the main form of money in Saint's Row IV. Cache (the menu option) is where you go to get your hourly cache income. You can get cache through activities, completing challenges, pickpocketing, just plain killing people, etc. Almost everything in the game will give some cache. But the main source will be the hourly income.
The bottom-center option in your hub menu is where you go to claim this hourly income. The bars on the icon will fill over the course of an in-game hour, and once full, will remain full and glow purple. Even though the meter isn't filling any more, the next run of the meter is still filling in the background; this can be seen by claiming your income, after which the meter will often not be completely full.
The amount you gain when you claim your hourly income is based on four things: specific upgrades, activities, targets, and stores. A picture will probably help.
The four sections are, going clockwise from the top: