Fate Grand Order: Epic of Remnant (FGO Project 1.5) is a continuation of Arc 1 in the form of four episodes before the start of Part 2.
The character that narrates the Trailer PV of Epic of Remnant gives that the story is to take place following the failure of Retrogression Canal・Genesis Light-Year (逆行運河・創世光年, Gyakkou-Unga・Sousei-Kounen?) (also the name of the Arc Drive used by Aozaki Aoko in Melty Blood) -- "a bit of detective-work" that serves as a prelude to the 2nd Project. The coming conflict will be premised upon something that Chaldea critically overlooked in their efforts to preserve the Common Sense of Man -- "the beloved legacy of the King of Magecraft."
Note : There are no specific order to the episodes, which means all 4 episodes can be done in any order.