What is the business code for a union officer ?

[JohnW15]

Based on your question, I'd suggest "813000" for "Religious, grantmaking, civic, professional, & similar organizations."

Sometimes the codes aren't specific enough for certain occupations, and you just have to go with what's closest. 

Please see "Principal Business or Professional Activity Codes" starting on page C-17 of the IRS' 2018 Instructions for Schedule C if you'd like to search for a more appropriate code.