'acct_id' vs. 'client_id' in Chat Reporting

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Can someone please explain to me the difference between the "acct_id" and "client_id" data elements in the chat_agent_sessions table?

 

For example, I've seen these two measurements both attributed to an agent in out-of-box chat reports and custom reports that I inherited, and they produce significantly different results:

1. sum_distinct(chat_agent_sessions.engaged_secs,chat_agent_sessions.acct_id)
2. sum_distinct(chat_agent_sessions.engaged_secs,chat_agent_sessions.client_id) 

 

In the reporting, the column with expression #1 has a header of "Total Chat Handle Time" and expression #2 has a header of "Total Time Engaged".