The distinction between USPS Priority Mail and First Class is the idea that the weight of the package determines how they are shipped.
For packages weighing over 13 ounces, which have a mailing value of at least $12.50, USPS uses priority mail service. If the package weighs less than 13 ounces, or if its value is less than $12.50, it can be sent via standard first-class mail service for cheaper rates.