.MIGRATEDAPLIBRARY File Extension
File TypeMigrated Aperture Library
What is a MIGRATEDAPLIBRARY file?
Library of digital photos created by Aperture that has been migrated to Photos, an image organization application bundled with OS X 10.10.3 and later; contains all of the images that appeared in the discontinued Aperture application along with the edits that have been made to them.
When Photos successfully migrates an Aperture library, the "aplibrary" file extension is changed to"migratedaplibrary" and Photos makes a hard link to all of the Aperture media assets, including original and preview image versions, which means that no additional disk space is taken up by the new library. Therefore, you do not need to delete the original Aperture library after migration.
NOTE: Photos performs a similar function with iPhoto libraries but uses the .MIGRATEDPHOTOLIBRARY file.