Relay interpreting is a variant of simultaneous interpreting.
Whenever more than two languages are spoken at a conference, more than one interpreting booth is required, as well. One team of interpreters each takes care of one language and works from it into the conference language and vice versa.
If the interpreter does not listen to the speaker's original presentation but relies on the interpretation that a colleague from another booth provides, the process is called 'relay interpreting'. Let's assume the conference languages are German, English, and Russian, and a speaker is currently talking in Russian. Now it may happen that the interpreter in the English booth does not speak Russian. Therefore he listens to the interpretation into German coming out of the Russian booth and takes this German version as the basis for his own rendering of what is being said into English.
If more than two languages are to be offered at a conference, the interpreting technique always must be simultaneous. Interpreters' booths and the corresponding conference technology are absolutely required, as well.