That appears to be the 'bundles' system, which is now the most common way to use PAYG. Using that system, each month is entirely independent of any other. So you can use Plan A for one month, Plan B for the next one, Plan A again for the 3rd month and Plan C for the 4th one. You can also stop paying altogether at any time, either for a short period or forever.
In practice though, most people set things up so that the same amount is taken from their bank account each month until they decide to change their plan or pull out of the arrangement altogether. Either way though, your service provider won't take any money from your account without your permission.