The orifice hoods are the little brass hoods that screw down on the gas jet that supplied the stove burners in a gas stove. Their purpose is to meter the correct amount of gas into the air-fule mixture that gets ignited at the burner ring. The diameters of the orifice vary with the BTU rating of the burner and the type of fuel being burned. Professor john63 provides us this handy listing of orifice metering hood size codes for LG gas cooktops and ranges:

Orifice size is determined by a letter stamped onto the orifice:

NATURAL GAS

LF: C

LR: D

RR: G

RF: H

CTR: D

BROIL: A

BAKE: B

LP GAS

LF: L

LR: M

RR: P

RF: Q

CTR: M

BROIL: D

BAKE: F

Source: LG Center Burners