I don't know what it is for, but it is a GPT feature. Since the G in GPT stands for GUID (Globally Unique Identifier), the PARTUUIDs might be the GUID of the corresponding GPT partition. The UUID, on the other hand, is from the filesystem.
The PARTUUID should remain unchanged if you format the partition to a different filesystem, as long as you don't modify the partition itself. I tested it on Arch Linux on a spare pen drive:
$ sudo blkid /dev/sdd1
/dev/sdd1: UUID="1e867d95-dd7f-4c97-aa3f-7ee4490744bc" TYPE="ext4" PARTUUID="a96b7ca5-f7f8-40c9-a35c-6d995787ab2f"
$ sudo mkfs -t ext4 /dev/sdd1
mke2fs 1.42.12 (29-Aug-2014)
/dev/sdd1 contains a ext4 file system
created on Fri Jul 10 20:51:19 2015
Proceed anyway? (y,n) y
Creating filesystem with 1954167 4k blocks and 488640 inodes
Filesystem UUID: 6c3393bf-aba1-4854-9815-f62818facfda
Superblock backups stored on blocks:
32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632
Allocating group tables: done
Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done
$ sudo blkid /dev/sdd1
/dev/sdd1: UUID="6c3393bf-aba1-4854-9815-f62818facfda" TYPE="ext4" PARTUUID="a96b7ca5-f7f8-40c9-a35c-6d995787ab2f"
$ sudo mkfs -t ntfs -Q /dev/sdd1
Cluster size has been automatically set to 4096 bytes.
Creating NTFS volume structures.
mkntfs completed successfully. Have a nice day.
$ sudo blkid /dev/sdd1
/dev/sdd1: UUID="2905A16E2CDB44E8" TYPE="ntfs" PARTUUID="a96b7ca5-f7f8-40c9-a35c-6d995787ab2f"
/dev/sdc1: LABEL="MUSIC_9" UUID="5A5B-2F15" TYPE="vfat" PARTUUID="7602dd12-01"
The UUID is actually the FAT32 serial number. I changed my serial number, and the PARTUUID did not change./etc/fstab
only shows the UUID and there is no PartUUID like there was in your Ubuntu 15.10? Does your current Ubuntu still have a PartUUID?/etc/fstab
showed PARTUUID.