So each of the vitamins comes in multiple forms called vitamers
Consuming different vitamers has different effects on the body
The amount of dosing suggested for two different vitamers of the same vitamin can be different
This is problematic regarding nutrition labels and the level of education given to the general public
e.g. when my mother had issues with most magnesium supplements like magnesium glycinate, @scalar recommended to try magnesium taurate which was much easier for her to digest since she has IBS issues. i don't think there's a specific term other than "form" or "compound" specific to what you're asking about.