In general, Geneious 6 and above can import data that has been exported in .geneious format from any newer version (e.g 7.0 and above), provided the appropriate option has been selected upon export. The Geneious Data Directory itself can not be loaded into an older version, data must be exported first.
For data stored in a Shared Database, each document can only be opened by versions of Geneious equal to or above the last version of Geneious that modified that document.
Geneious 5.6 and earlier are not back-compatible, e.g you cannot open .geneious files created in Geneious 6 in Geneious 5.6.
Patch releases are always cross compatible, e.g a database from version 8.1.7 will open in all 8.1.x versions.