What is Channel?

Channel is an ability... 
that does absolutely nothing! How could this possibly be useful? 
If you have checked out Zoizite's Trigger Spell tutorial or have ever 

worked with trigger spells before, 
then you already know how.

The Channel ability has several fields that allow you to customize exactly how the spell will be cast, giving you much greater flexibility than using any other spell as a base. First off, almost all Art fields can apply in some way to Channel (Lightning Effects is an exception).

Let's take a look at each of the Data fields:
Art Duration: Time interval for showing the Art - Effect (or maybe this depends on the targetting behavior?). This applies only when the spell is being channelled.
Base Order ID: This field allows you to give multiple versions of Channel to a single unit. Change the Base Order ID for each Channel spell, and make sure that none of them overlap with any other spells that a unit might have.
Disable Other Abilities: Disables other abilities when this spell is being channelled (True/False).
Follow Through Time: Duration of the spell. The unit will channel until interrupted for up to the duration of the spell. A value of "0.00" denotes a non-channelling spell.
Show UI: When set to False, no button will appear in the UI. When set to True, the button appears. This field has other hidden tricks up its sleeve. See the next section for more details.
Target Type: Determines the targetting behavior of the spell.
Instant (No Target)
Unit Target - also applies to Destructibles (I think?)
Point Target
Unit or Point Target - Can either target a Point or the Position of Target Unit (or Destructible?) of Ability Being Cast. Considered a Point Target spell when used in triggers (Ex. Shockwave)

The rest of the Stats fields should be familiar. The Area of Effect field is meaningless... or is it? Read on!

New Channel Features

When Blizzard implemented the 1.13 patch, they added a good deal of functionality to a good number of spells. As you may already know, one of the most significant ones was allowing a unit to have multiple versions of the Channel ability. There is another bit of functionality that they added but forgot to expose in the editor. They improved the "Show UI" field to do more than just show or not show the ability in the UI.
Instead of a simple boolean, the "Show UI" field should actually be an integer field. It involves a series of bitflags (basically a bunch of booleans crammed into one field).

Here are the abilities of each bitflag (written in decimal for simplicity's sake):
1 - Visible - Show UI
2 - AoE Targetting Image (!)
4 - Physical Spell (Like Ensnare, maybe?)
8 - Universal Spell (Anyone have any examples?)
16 - Unique Cast - Only one of a group of casters will cast the spell at a time
(Note: Default is "Magical Spell")

To get any combination of effects, just add up the values of each effect. For example, if you want a spell to have Unique Cast, show the AoE Targetting Image, and be Visible, then the value for the field would have to be 16 + 2 + 1 = 19.

Great! But... when you try to edit the Show UI field, all you get is a True/False option, even with a shift-click! How annoying.

Fear not, there's a very simple way of enabling this functionality. You have to tweak the AbilityMetaData.slk file to change the Boolean data type to an Integer. You won't have to worry about editing anything; the modified file is attached to the previous post.

After you get that file, create a directory in your Warcraft III folder, called "Units". Place the file in that folder.

For easy reference of what the bitflags are, you will also want to throw in a hint for that field. For this, create another directory in your Warcraft III folder, this time called "UI". Download the WorldEditStrings.txt from this post, and place it in that folder.

Now, if you have the World Editor open, you will need to restart it for these changes to take effect. Check out the Channel ability, and you will see an editable integer field with a hint showing the values of all bitflags. Enjoy customizing your custom spells to an even greater degree than before!

Note: Putting these files in their respective folders will have no adverse effect on your World Editor. You do NOT need to import any files into your map, whatsoever, for your modified Channel abilities to work properly.

As a final note, when the 1.15 patch is released, this bug will probably be fixed and all fields of Channel will be able to be viewed properly. At this point, you will need to delete the Units and UI folders that you created. But don't hold your breath!

Good luck with your custom spells!