A trademark application number is a unique eight-digit number that is assigned to a trademark by the USPTO once the application is filed. The trademark application number contains a two-digit series code followed by six additional digits generated in chronological order based on when the application was filed.

What the numbers means in a trademark serial number.
Series versus serial number in trademark filings.

As a general rule, the two-digit series code is determined based on the year the application was filed. For example, the series code “75” was used for applications filed on or after October 1st, 1995 until the next series code “76” was implemented for applications filed on or after March 20th, 2000.