A more thorough explanation:

A batch number is a numerical designation assigned to a group of applications for patent or trademark registration filed in the U.S. Patent and Trademark Office. It is also known as a series code.

For example, if an application number is 09/445,323, then 09 is the series code, and the application is the 445,323rd application in that batch.

Series codes are assigned for a group of applications filed during a particular period. Nonprovisional patent applications are assigned series codes from 01 to 10, depending on the period during which an application was filed. The series code for design applications is 29, for provisional applications 60, and for reexamination proceedings 90. For trademark applications, the series code usually begins with numbers from 70 to 75.

For instance, if a company files ten patent applications in a particular period, the U.S. Patent and Trademark Office assigns them a batch number or series code from 01 to 10, depending on the order in which they were filed. This helps the office to keep track of the applications and process them efficiently.