Control Area ICAO

[Engineer]

Defined as controlled airspace extending upwards from a specified limit above the earth

Question

What is the minimum specified limit above the earth?

i.e the lower limit of a control area shall be a height not less than ???

Can't find any reference in the ATC section of the Jepps so would be grateful for any info.

By the way I know it is not zero because then it would be a Control Zone :*

[Tinstaafl]

The limit is whatever the regulator specifies for that geographical location ie the altitudes they publish in their AIP/charts that say "Here there be CTA between altitude xxx & altitude yyy".