Interpreters talk about active and passive languages.
An active language is a language the interpreters speak that delegates can listen to.
A passive language is a language the interpreters understand that is spoken by the delegates.
A meeting with a 24-24 language regime has 24 passive and 24 active languages. In the European Union institutions this means that all the official languages are interpreted into all the official languages. Such a regime is called complete and symmetric.
Reduced regime means that interpretation is provided from less than the full number of official languages.
Symmetric regime means that delegates can speak and listen to intepretation from the same languages.
In an asymmetric regime, you can speak more languages than you can listen to. When we say a meeting has an 15-3 regime it means that delegates may speak 15 official languages but that interpretation is only provided into three, for example English, French and German.
A meeting with an EN-FR-DE-IT-EL - EN-EL is what we call a 5-2 meeting where delegates may speak English, French, German, Italian or Greek but interpretation is provided only into English and Greek. Such a meeting has a reduced asymmetric regime.