This field is used as part of the account code calculation - for non-major gift donations we use channel, for major gifts we use a major-gifts specific code. In theory there are 2 major gifts account codes - for restricted and unrestricted but I'm having trouble find examples of recent restricted gifts & the are probably not relevant to online donations.
There is also endowment - still getting more clarification on handling here (from Tiffany) but in theory a donation can be endowment AND major gifts & so the final accounting calculation takes into account the 3 fields