How the Heat system works:
Each weapon has a bounty level that starts from 1. Once you have heat equaling or exceeding that bounty level for that weapon, you get new rewards. Once you clear with that weapon at that bounty level, the bounty level rises by 1.
For example:
Starting with Bounty level 1, you take one Heat. This allows you to get the unique rewards from each boss (assuming you haven't beaten that boss/bounty/weapon combo yet), rather than just a pile of Darkness. Once you clear the game, that weapon goes up to Bounty level 2, and you now need two Heat to get new rewards.
Also starting with Bounty level 1, you find low heats boring, so you skip to Heat 5. However, you still only get the rewards from Bounty level 1. Once you clear, the bounty level rises to 2 for that weapon, which you can then use the same 5 Heat to obtain.
Each weapon has a bounty level that starts from 1. Once you have heat equaling or exceeding that bounty level for that weapon, you get new rewards. Once you clear with that weapon at that bounty level, the bounty level rises by 1.
For example:
Starting with Bounty level 1, you take one Heat. This allows you to get the unique rewards from each boss (assuming you haven't beaten that boss/bounty/weapon combo yet), rather than just a pile of Darkness. Once you clear the game, that weapon goes up to Bounty level 2, and you now need two Heat to get new rewards.
Also starting with Bounty level 1, you find low heats boring, so you skip to Heat 5. However, you still only get the rewards from Bounty level 1. Once you clear, the bounty level rises to 2 for that weapon, which you can then use the same 5 Heat to obtain.