Compare the allowances
Credits measure AI usage, rather than a fixed number of messages. Different models and amounts of text can consume different amounts. Pro does not mean unlimited included messages or free additional usage. How credits work explains the unit and how separate allowances are used.
On-demand usage means paid usage after the included credits are spent. Pro can stop at its allowance, or continue with this additional usage if enabled. Turn on overages and set a cap explains Fixed, Unlimited and Disabled.