A record can have a creation date and a changing date. According to Gedcom, only the changing date is standard compliant (CHAN). However, some programs also store the creation date (f.i. Gen_Pluswin with _CREA).
An underscore “_” before the tag name means that it is not a standard GEDCOM tag. Therefore if it is included in webtrees it most likely comes from a program that 1) the development team knows that another program sends it in a GEDCOM load 2) probably does not own a copy of that program and therefore is not 100% sure of its meaning.
MP is probably correct in its use, but it could be something else!