_id field - Unique identifier of any record inserted into collection

Data accepted by _id field
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db.users.save({_id:1})
db.users.save({_id:3.14})
db.users.save({_id:"Hellow World"})
db.users.save({_id:ISODate()})
db.users.save({_id:{A:'x', b:1, c:3.14}})


Apart from an Array value,  you can pass any value to an _id field