Answer
There are two parameters that have effect on ploop containers' second-level quota functionality: quotaugidlimit and jquota.
quotaugidlimit parameter for ploop containers is a switch: "0" value turns off the quota support, while any positive value turns it on. There are no more limits, opposed to PVC 4.x versions, as ploop containers possess their own ext4 file system and may have their own mount options. Quota accounting may be controlled by the quota utilities, shipped with the OS (edquota/repquota).
For modern operating systems (such as centos-6, debian-6, ubuntu-12 and later) there is an additional capability - so-called journalled quota. It is always enabled by default for all ploop containers and can be controlled by --jquota key:
[root@pcs ~]# vzctl --help | grep jquota
[--quotatime <N>] [--quotaugidlimit <N>] [--jquota <on|of>]
With both jquota and quotaugidlimit enabled, mount options usrjquota and grpjquota appear in /proc/mounts, for example:
/dev/ploop30913p1 /vz/root/101 ext4 rw,relatime,barrier=1,stripe=384,data=ordered,balloon_ino=12,pfcache_csum,pfcache=/vz/pfcache,jqfmt=vfsv0,usrjquota=aquota.user,grpjquota=aquota.group 0 0
If the OS doesn't support jquota or it is turned off, but quotaugidlimit is turned on, options usrquota and grpquota appear insted, for example:
/dev/ploop10225p1 /vz/root/102 ext4 rw,relatime,barrier=1,stripe=384,data=ordered,balloon_ino=12,pfcache_csum,pfcache=/vz/pfcache,usrquota,grpquota 0 0