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Must daemons be summoned to the battlefield or can I just field them as normal? (Of course I have to pay the point cost for them either way)

You can field them as normal in an appropriate detachment (so must have a faction of either CHAOS DAEMONS or a god alignment, like NURGLE, SLAANESH, etc)

Note that in the latter, if you include marked units of other CHAOS factions (ie, Plague Marines) you lose detachment bonuses from Death Guard and Chaos Daemons.
If you mix NURGLE daemons with SLAANESH daemons in one Chaos Daemons detachment you lose daemonic locus abilities.

Thanks.