Fixed costs apply monthly and is a constant amount of credits.
This is billed even for a monthly as an hourly deploy.
Usage costs are the credits you consume either monthly or hourly with the Desktop(s).
In a monthly deploy, the Usage cost is a fixed amount of credits; It's based on an all-you-can-eat model with a fixed monthly price.
In a hourly deploy, the Usage cost is based on the amount of hours your Desktop is powered on.
Each started hour is billed as 1 hour; even if it's powered on for only 1 minute, 20 or 59 minutes.
See screenshots below for an example of a deploy.