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Both scripts work on managing budgets, however, one focuses on avoiding overspending, and the other one focuses on ensuring you reach a target monthly spend for your budget.
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The Pause When Things Spend Too Much script helps you ensure you never overspend your budget. It checks if an item is exceeding the budget and does two things to avoid going over: it pauses the entity and notifies you over email. This allows better control over your budget, and you can set the script to re-enable the entity at the beginning of the next budget period.
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The Reach Target Monthly Spend script, on the other hand, will adjust the budgets to meet a target spend. Instead of pausing the entity once the budget is exceeded, it modifies campaign-level budgets to reach your target spend, helping you complete the full budget allocated for the month.

The shared budget option is used when you are using the same budget across multiple campaigns. Select this to manage a shared budget, and leave it unselected if you want to manage the budget of an individual campaign. 

We use this setting because campaigns and shared budgets don't have to have unique names in Google Ads, so when the name you entered in the settings matches both a campaign and a shared budget, this tells the script which one you would like to manage automatically.

This script works by checking for budget names and pulling in data for that budget, with the corresponding setting on whether it's shared or not. But if the budget association changes, for example, mid-month, the spend from one budget won't be transferred to the other. 

For example, if you have a campaign with a shared budget, and halfway through the budget cycle, you change it to a campaign budget, the script won't consider the spend the campaign had before that moment. This is because the spend the campaign had can't be transferred from the shared budget to the campaign budget.

If you need to change from a campaign budget to a shared budget, we'd recommend doing it at the beginning or at the end of the budget cycle to avoid any confusion and to keep the spend projected in the script as precise as possible. 

Yes, both Budget Scripts can be used for Performance Max campaigns. But you'll need to make sure they run on the New Google Ads Scripts Experience. You can read more about this here.

You can track data for these campaigns using these scripts but are restrained from taking any sort of action, like updating the campaign's budget or enabling/pausing these types of campaigns. The restriction is from the API side for this specific campaign type.

Due to API restrictions, it's not possible to track or apply changes to smart campaigns through the script.

It's enough if you change it in the Optmyzr settings. Once the script is downloaded and running on the Google Ads account, there's no need to re-install it.

The same applies if you use a spreadsheet in the script settings: you can just simply change the budget amount in the spreadsheet, and the script will be able to pull the updated amount the next time it runs.