When a user shares their professional voice clone with the Voice Library, they can choose to set a custom rate, rather than the default rate, for financial rewards. When you use a voice with a custom rate to generate audio, this will have a credit multiplier in effect, meaning that it will cost more credits to generate with this voice.
If a voice has a credit multiplier in place, this will be displayed as a tag in the Voice Library:
It will also be shown as an attribute in the detailed view for the voice in My Voices:
In Speech Synthesis, you will see a notification that the voice has a credit multiplier in place:
The credit multiplier that applies for generating audio is based on the custom rate chosen by the voice owner as well as the subscription plan of the user of the voice.