From what I've seen on other GDTF Descriptions, where ever you have a channel which has more than one linear value (e.g. colour presets, or chase modes) - you add a channel set.
So you define a logical channel, and a channel function, then at the bottom right of the builder (you might have to scroll down on the page), you have the option to add a channel set.
This is taken straight from the Robe Robin T1 as an example :
<LogicalChannel Attribute="AnimationWheel1SelectEffects" DMXChangeTimeLimit="0.000000" Master="None" MibFade="0.000000" Snap="No">
<ChannelFunction Attribute="AnimationWheel1SelectEffects" DMXFrom="0/1" Name="AnimationWheel1SelectEffects" OriginalAttribute="Effect wheel animations" PhysicalFrom="0.000000" PhysicalTo="1.000000" RealFade="0.000000">
<ChannelSet DMXFrom="0/1" Name="No animation" PhysicalFrom="0.000000" PhysicalTo="1.000000" WheelSlotIndex="0"/>
<ChannelSet DMXFrom="8/1" Name="Macro 1 (Focus=94)" PhysicalFrom="0.000000" PhysicalTo="1.000000" WheelSlotIndex="0"/>
<ChannelSet DMXFrom="10/1" Name="Macro 2 (Focus=84)" PhysicalFrom="0.000000" PhysicalTo="1.000000" WheelSlotIndex="0"/>
<ChannelSet DMXFrom="12/1" Name="Macro 3 (Focus=87)" PhysicalFrom="0.000000" PhysicalTo="1.000000" WheelSlotIndex="0"/>
<ChannelSet DMXFrom="14/1" Name="Macro 4 (Focus=69)" PhysicalFrom="0.000000" PhysicalTo="1.000000" WheelSlotIndex="0"/>
<ChannelSet DMXFrom="16/1" Name="Macro 5 (Focus=82)" PhysicalFrom="0.000000" PhysicalTo="1.000000" WheelSlotIndex="0"/>
<ChannelSet DMXFrom="18/1" Name="Macro 6 (Focus=82)" PhysicalFrom="0.000000" PhysicalTo="1.000000" WheelSlotIndex="0"/>
<ChannelSet DMXFrom="20/1" Name="Macro 7 (Focus=72)" PhysicalFrom="0.000000" PhysicalTo="1.000000" WheelSlotIndex="0"/>
<ChannelSet DMXFrom="22/1" Name="Macro 8 (Focus=86)" PhysicalFrom="0.000000" PhysicalTo="1.000000" WheelSlotIndex="0"/>
<ChannelSet DMXFrom="24/1" Name="Macro 9 (Focus=78)" PhysicalFrom="0.000000" PhysicalTo="1.000000" WheelSlotIndex="0"/>
<ChannelSet DMXFrom="26/1" Name="Macro 10 (Focus=102)" PhysicalFrom="0.000000" PhysicalTo="1.000000" WheelSlotIndex="0"/>
<ChannelSet DMXFrom="28/1" Name="Reserved" PhysicalFrom="0.000000" PhysicalTo="1.000000" WheelSlotIndex="0"/>
</ChannelFunction>
</LogicalChannel>
You can see that in a DMX channel that had multiple options, a channel set is used to define the different modes.