Nested document different syntaxes

Working with Data
1

Dear colleagues,
Can anyone help me with confuse
Why this code works (and return 1 result):
db.inventory.find( { "size.h": { $lt: 15 }, "size.uom": "in", status: "D" } )

And this one didn’t (return 0 result):

db.inventory.find( { 
    "size": {
        "h": { $lt: 15 },
        "uom": "in"
    },
    "status": "D" 
})
2

Field based query vs object based query.

See Embedded document query not working! - #2 by steevej