SQL Server: What's the Difference Between a 'Singleton Lookup' and a 'User Lookup'?

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.