Microsoft describes the `[user_lookup]` counter in `dm_db_index_usage_stats` as the 'Number of bookmark lookups by user queries.'  It describes the `[singleton_lookup_count]` in `dm_db_index_operational_stats` as the 'Cumulative count of single row retrievals from the index or heap.'  This is not at all clarifying to me.  Could someone provide a clearer definition/differentiation of what these two values are tracking?

The question stems from an index observation (800m row table) today where the current `[user_lookup]` value on the Cx was around 800 and the `[singleton_lookup_count]` was over 1 billion.  They're obviously counting much different things and I need to understand what they are and whether a difference of that magnitude could be indicative of a problem.