Some work done in Lightroom 6 cannot be transferred back to LR 5. Other work can be transferred.
Most but not all metadata, and most but not all edits, can be written to XMP by LR 6/LR CC and then read by Lightroom 5. Edits that take advantage of new features in Lightroom 6/Lightroom CC will not be recognized by Lightroom 5. Metadata that cannot be written XMP of course cannot be transferred from LR 6/LR cc to LR 5.