Still don't know why this green bar appears with a "reset" option. What is it resetting? Why has the developer left it in when there is no clear explanation what it is doing
You are creating a "subset" of UTXOs (unspent outputs) to spend from... you can "shift" or "ctrl"-click multiple addresses/coins in the "Addresses" or "Coins" tabs and add them to that subset... It will then tell you how many outputs you have available in the selected subset (and also the total your wallet actually contains), and the maximum amount of BTC you can now spend
Clicking "reset" simply clears this "subset" and will allow you to spend the max balance of the wallet from ALL of your addresses.
In this example, I've selected 1 output from the 45 available in my wallet... and can only spend 0.1 BTC from my total wallet balance of 7.2+BTC (yay for TestNet!
):
In this example, I've now selected 8 outputs from the 45 total... and can spend a max of 2.10954629 BTC:
Clicking "reset" simply means I can spend everything, but the wallet will then decide automatically which addresses/outputs to use.