Hi Jessica,
If the first PR consumes 90 of the Budget and when the second PR if you try to put more 30, system will stop you to save this PR as the Budget is exceeded. At this stage the cost planned in PR or PR converted PO it is called as commitments.
It will consider if PR is converted to PO also, when the goods issue occurs, this cost will be changed from commitment to actuals.
You need to config the system to give a warning or error message when the Budget is exceeded.
Regards
Terence