I believe this is covered in the UC Surface manual:
And also mostly covered in another question here:
...though it looks to refer to an older UC Surface version, so buttons are a little different, but the concepts are the same.
You have FX mix buses A, B, C, and D (which are set to Post by default BTW, so that FX mixes are post-fader, as they should be in most situations), and those are mix bus sends to the internal FX processors (A and B are reverb, C and D are delay).
You do an FX mix (the faders set the channel send level to that FX mix) and you then turn up the FX bus Send (the Flex Fader) to control the output level of the FX bus that's sent to the FX processor.
The FX processors then return to each mix (Main Mix, Aux Mixes). You have FX Return faders in each mix, which you can see by scrolling channels all the way to the right.
Normally the FX buses are set to Post, meaning that the levels of any channels sent to the FX bus are dependent on both the Main mix faders and the FX mix faders (i.e. if a channel is turned down or muted in the Main mix, then it won't be sent to the FX mix/processor, even though the channel is turned up in the FX mix). This way when you turn down a channel in your Main mix, it doesn't continue feeding the FX processor (which is normally returning to your Main mix), resulting in only the 100% wet effected signal for that channel continuing to come through.
If you have FX buses set to pre1 or pre2, that means that the levels of any channels sent to the FX bus are controlled entirely by the FX mix faders (i.e. even if the channel is turned down or muted in the Main mix, if the channel is turned up in the FX mix, it's being sent to the FX processor).