Live like it's Christmas every day - Santa Stephen
Any tag with an underscore "_XXX" is traditionally (and in terms of the standard) referred to as a custom tag. webtrees does have a very few designed to assist in the recording of webtrees-specific tags, but most are what are termed "generally recognized, widely used custom tags". In particular, you have denoted several: _MARNM (married name), _MILI (military), _AKA (Also Known As), etc.
When exercising the export or download of a GEDCOM, webtrees gives you the opportunity to strip out any webtrees specific tags, although if you intend to use a GEDCOM with webtrees, that should not be the option you choose.
Santa Stephen the Fabled Santa
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