Origin: Latin suffix -al
Supplemental has 4 different meanings across 1 category:
Something that supplements or adds to.
"The supplemental report provides additional data not included in the main document."
In plain English: A supplemental item is something extra added to what already exists to make it more complete.
"The supplemental was an extra payment added to her regular salary."
functioning in a supporting capacity
"the main library and its auxiliary branches"
added to complete or make up a deficiency
"produced supplementary volumes"
Acting to supplement.
"The supplemental lecture provided extra details that were not covered in the main textbook chapter."
The word supplemental comes from combining the root supplement with the suffix -al to mean something that adds to or completes a whole. It entered English as an adjective describing material provided in addition to what already exists.