Zero linking is an option for periodic references, when one external device switches two logical channels. For example, this might occur if you are using a two-channel gas analyzer that simultaneously reads gas streams from two separate metabolic chambers, each of which is recorded on its own logical channel.
In this situation the 'primary' channel automatically records zero when not 'connected' to the switched instrument. However, it is useful for the second logical channel to also record zero when not 'connected.' Zeroing avoids confusion during analysis (but note that all channels, switched or not, record data during reference periods).
To 'link' another channel to a primary channel, click the appropriate button to the right of the primary channel's label.