Learn English – single word that means “under the table”

When you speak of capitalizing a word, that normally means to capitalize only the first letter. If you were to say "capitalize all the words that appear in the table headers", I believe this would get your meaning across fine (and as always in specifications, a little example goes a very long way).

The only other likely interpretation, putting it in all caps, is not normally expressed that way; usually one would have to say "put the words that appear in the table headers in all caps" to communicate that.

Affix

an additional element placed at the beginning or end of a root, stem, or word, or in the body of a word, to modify its meaning.