From Wikipedia 'Union Dues' page:
"Assessments are generally one-time-only payments made by the union member to the union to cover a special program or activity. These special programs may or may not be ongoing, and may or may not operate for a limited time or in a limited fashion."
I'm not sure what it is for in your local or mine honestly. I'm sure if you called your local and asked they could give you an explanation of it. I never worried about it for $26 a year, but I don't blame you for wondering.