Credits are charged anytime you generate new text-to-speech audio files using a non-free (standard or high-quality) voice. This is done when you hit the play button to hear the audio, or when you export the audio to files.
If you don't change the text, voice, or other settings (e.g. rate, pitch, format) then you can play or export the same files as many times as you want on any device with the same iCloud account without being charged because the audio files are stored in iCloud and don't need to be regenerated. However, if you change the text, voice, or other settings, you will be charged again the next time you attempt to play or export the audio files.
If you see a number of credits to be charged in the interface, that is the amount you will be charged if you play or export the audio files. If the audio files have already been generated and can be exported again without charge, the interface will show "Purchased" which means you will not be charged for any more plays or exports.
Exported audio using any of the (paid) standard or high quality voices are licensed for commercial use.
However, audio created using the free, built-in voices are not licenced for commercial use since these voices are owned by Apple and included in iOS or macOS for personal, non-commerical use only. See Apple's Software License Agreements for more details.