USPS tracking numbers come in different formats depending on their mail classes. Most domestic USPS tracking numbers are 22 digits long and start with the number 9. The first four numbers in a USPS tracking number reveal the type of mail service the package is using, which is also known as the service code. For example, tracking numbers that begin with 9400 denote that it is the classic USPS tracking code, and those starting with 9270 mean that it is from a Priority Mail Express. The first two numbers of the tracking number can also indicate the type of mail service used. For instance, tracking numbers that start with 92, 93, 94, or 95 are for commercial customers/mailers, while 94 is for labels printed online and 95 is for labels printed at the counter/retail.

It is important to note that international packages may have a different format. For example, USPS international tracking numbers begin with a combination of letters, and those different letter combinations depend on the type of service you are shipping and whether the packages are fully trackable.