To "Elite" a plane you must research all its modules AND any planes directly above it. Some planes lead to 2 other planes and BOTH must be researched to elite the original plane.
For example at tier 6 the IL-2(t) cannot attain "Elite" status unless all of its modules AND both the IL-8 and IL-10 are researched. You absolutely do not need to own a plane to elite it.
So what good is elite status? You can check a box that makes all the xp you earn go directly to training your crew or you can spend gold to convert your elite plane's xp to free xp which can be used toward any plane.
You can read more about it here: https://na.wargaming.net/support/kb/articles/234
'Customize Your Carousel' thread - LINK
Custom skins for I-16(L), LBSh, & IL-20 - LINK