Link menu: Determines which effects are linked. If you link effects, parameter changes apply to all effects in the linked group. You can choose between five groups: A, B, C, D, and E. Channels set to none (“-”) operate independently. Each group features its own tab. When you link channels, keep the following in mind:
If you link two channels as a stereo pair, the left channel is always assigned as the master.
If one or more channels are added to an existing group, the new members adopt the values of the group.
If you assign several channels to an unused group, the setting of the (front) left channel is used for all group members.
Bypass button: Click to bypass the channel: it is routed around the effect. This is useful when individual channels should be exempt from processing by a particular plug-in.
Note: If the Bypass button of a grouped channel is clicked, all channels in the group are routed around the effect.
Logic Pro User Guide: PDF
Logic Pro Effects: PDF