Enhance help for each reporting field so that users can check valid values for each field easily by using "-o help <field_name>". Such field-specific help would include brief description of the field (we actually have this in the "-o help" already more or less) and stating possible valid value ranges this field can display, including undefined values and any synonyms which are recognized for -S|--select criteria. For example: lvs -o help lv_role ...would display all possible LV roles. This way, users know what values they can expect and they can add proper checks in their scripts.
Also, while in this area, try making it easier for people to search for fields since we have lots of them now and they didn't even fit into one usual screen (80 lines) anymore (currently there are 191 lines printed so it scrolls most of the time). So try adding support for wildcards like "-o help cache_*", also searching through substrings (both in field names and descriptions) or any other improvements that may help users to navigate through the list of fields which LVM can report.