What is ear simulator coupler / 2ccm coupler?

Greetings,
in many hearing aid datasheets i have seen these two kind of records.
what exactly are these?
thank you

Hearing aids are tested/calibrated with a fake ear cavity made of metal.

The 2cc model is most commonly used to create reference data … but is crude.

There is a more complex metal cavity (IEC) which is nearer the human ear acoustics but is less often used and is more expensive.

Neither will have the acoustics of YOUR ears!

thank you very much for your feedback !
So i shouldnt trust these data for choosing hearing aid?
is there any other record i could check , to see if the hearing aid will give me enough volume in the freequencies i need most?

The IEC data is probably close enough.

The variability in your ears is probably big enough to mask any ‘errors’ resulting from using coupler data.

IEC is ear simulator coupler?