Question - What are the ISID folders in the project data folder?
Answer - The ISID folders are used to hold non-default images that are used for things like export sets and bulk printing. Default images are used wherever possible. But consider the case of an export set with images and branding. The product cannot brand the default images for in an export set, since (as opposed to production sets), the original objects are used. If the default images were branded, then the branding would appear when viewing the exported objects later in Review. For that reason, the images to be branded are copied to an ISID_xxx folder that corresponds to the export set, and the copied images are branded and exported. The space can be freed by deleting the export sets.
Bulk printing also brands temporary images in ISID_xxx folders, but they should be deleted after they are used.
Overview - ISID, project data folder, case data folder, disk space, drive space