For the bulk of the last two decades, 20th Century Fox churned out a great many Marvel films with the characters they had the theatrical rights for, namely the X-Men and the Fantastic Four. These attempts were met with varying degrees of success from both critical and financial standpoints.
Ever since The Walt Disney Corporation's acquisition of 20th Century Fox and all of its entertainment assets, some of the Fox Marvel movies have gradually worked their way into the Disney+ roster.
Until recently, these films were displayed on the streaming service alongside Marvel Studios' MCU movies in the same category, which could've potentially caused a good deal of confusion from casual fans or Disney+ users who are new to the Marvel brand.
Disney+ has remedied this by now denoting all of Fox's Marvel films which currently reside on the streaming platform with the label "Marvel Legacy":
The Marvel Legacy section itself includes: X-Men (2000), X2: X-Men United, Fantastic Four (2005), The Wolverine, X-Men: Days of Future Past, Fantastic Four (2015) and X-Men: Apocalypse.
WHAT THIS MEANS
As one can probably see, it's much more simple to see which films belong to which studio. Additionally, the Marvel Studios films on Disney+ have now received the heading of "Marvel Cinematic Universe'.
This all should make it far less confusing for curious Disney+ users to figure out which movies belong to which continuities and maybe even discover something new that they haven't yet seen.