This statistic is not officially recorded by the NFL, but some teams record it and make the information public. A tackle assist is awarded if a player is tackled by at least two players. The determination of which player is awarded a solo tackle, or if two players deserve an assist with no solo tackle is up to the statistician employed by the home team.
In most cases, a tackle assist is awarded if a second player sufficiently helped tackle the opposing player with one of his teammates. Piling on or preventing further forward progress is generally not credited as a tackle assist. Since the statistic is unofficial, it is measured inconsistently and subjectively across the NFL. Most statisticians will not award more than two assists on a given tackle, and will only give out one assist if a solo tackle has been recorded.