Calendar - what are session_open and session_close?

https://alpaca.markets/docs/api-documentation/api-v2/calendar/

For start of 2020-01-01 the response form (with formatting added for readability) is:

[
  {
    "date":"2020-01-02",
    "open":"09:30",
    "close":"16:00",
    "session_open":"0700",
    "session_close":"1900"
  },
  ...
]

The properties date, open and close are documented.

What do session_open and session_close represent?

Perhaps the intent is to represent extended hours at some point? The current values don’t align with the current extended hours with Alpaca though.