And, AFAICT, it isn't present in any LVM manpage:
$ zgrep tags /usr/share/man/man*/*lvm*
/usr/share/man/man5/lvm.conf.5.gz:See \fBtags\fP configuration setting description below.
/usr/share/man/man7/lvmreport.7.gz:Let's display one one more field in addition ("lv_tags" in this example)
/usr/share/man/man7/lvmreport.7.gz:# lvs \-o+lv_tags
/usr/share/man/man7/lvmreport.7.gz:# lvs \-o+tags
/usr/share/man/man7/lvmreport.7.gz:# lvs \-o name,size,origin,snap_percent,tags,time
/usr/share/man/man7/lvmreport.7.gz:# lvchange \-\-deltag test \-S 'tags = test'
/usr/share/man/man7/lvmreport.7.gz:whole list strictly, its subset or intersection. Let's take "lv_tags"
/usr/share/man/man7/lvmreport.7.gz:tags field. We're using { } to denote that we're interested in subset
/usr/share/man/man7/lvmreport.7.gz:# lvs \-o name,tags \-S 'tags={tagA}'
/usr/share/man/man7/lvmreport.7.gz:# lvs \-o name,tags \-S 'tags=tagA'
/usr/share/man/man7/lvmreport.7.gz:# lvs \-o name,tags \-S 'tags={tagA,tagC,tagD}'
/usr/share/man/man7/lvmreport.7.gz:# lvs \-o name,tags \-S 'tags={tagA || tagC || tagD}'
/usr/share/man/man7/lvmreport.7.gz:# lvs \-o name,tags \-S 'tags=[tagA]'
/usr/share/man/man7/lvmreport.7.gz:# lvs \-o name,tags \-S 'tags=[tagB,tagA]'
/usr/share/man/man7/lvmreport.7.gz:# lvs \-o name,tags \-S 'tags=[tagA || tagC || tagD]'
/usr/share/man/man7/lvmreport.7.gz:# lvs \-o name,tags \-S 'tags=""'
/usr/share/man/man7/lvmreport.7.gz:# lvs \-o name,tags \-S 'tags!=""'
/usr/share/man/man8/lvm.8.gz:.B tags
/usr/share/man/man8/lvm.8.gz:Display any tags defined on this host.
/usr/share/man/man8/lvm.8.gz:Physical Volumes are considered to match if any of the listed tags is
ProblemType: Bug
DistroRelease: Ubuntu 20.04
Package: lvm2 2.03.07-1ubuntu1
ProcVersionSignature: Ubuntu 5.4.0-21.25-generic 5.4.27
Uname: Linux 5.4.0-21-generic x86_64
NonfreeKernelModules: zfs zunicode zavl icp zcommon znvpair nvidia_modeset nvidia
ApportVersion: 2.20.11-0ubuntu24
Architecture: amd64
CurrentDesktop: i3
Date: Tue Apr 7 15:54:40 2020
InstallationDate: Installed on 2019-05-07 (336 days ago)
InstallationMedia: Ubuntu 18.04.2 LTS "Bionic Beaver" - Release amd64 (20190210)
SourcePackage: lvm2
UpgradeStatus: Upgraded to focal on 2019-11-15 (144 days ago)