Referring to the FP8 ManualL
https://pae-web.presonusmusic.com/downloads/products/pdf/FaderPort8_OwnersManual_EN_21122016.pdf
Page 24 Section 4.6.3:
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Like Edit Plug-ins, engaging Sends offers two different modes:
Mix View and Channel Focus.
To engage Mix View, press Sends once. In this mode, the Faders will control the first send for every channel and the Select button will control the first send Mute for every channel.
To engage Channel Focus, press Sends twice. In this mode, the following four parameters are available for Send Slots 1 and 2:
• Send Destination.
Use the Fader to select the Bus. Press the Pan/Param encoder to commit the Bus selection. Fader 1 controls this parameter for Insert Slot 1. Fader 5 controls this parameter for Insert Slot 2.
• Send Level. Use the Fader to control the Send level. Fader 2 control Insert Slot 1’s send level. Fader 6 controls Insert Slot 2’s send level.
• Send Pre/Post selection. Use the Fader to switch between pre- and post-fader send positions. Fader 3 sets this position for Insert Slot 1. Fader 7 sets this position for Insert Slot 2.
• Send Active/Mute. Use Faders 4 and 8 to enable / disable the send for Insert Slot 1 and 2 respectively.