When I built "webapps" in the 90s, they were a collection of frames. You could just do everything as one page, but there were some neat things you could do by dividing the page up into frames + internet connections were not very good so if you had something you needed to update a lot, it was easier to make it it's own .html and server it in an iframe.

Because you could "NameAFrameAnyThingYouWanTED" they made some magic targets, _blank window to open a blank window (we have tabs, then we did not), _self to open in the same frame or window, _parent to open in the parent frame, _top to break out of frames entirely (this is how you would bypass the ad some free .coms would inject into your site)

When I read the html spec in the 90s all it says was you can name a frame anything except don't use _ as _ is reserved for special frame types.