Replying to a thread from 2011, but if it can help more users, why not :

You are mixing revision on the title, and revision of the hack.

About the Title revision:
--------------------------------
The "Title revision" is what nintendo use to determine if there's a need to update the console.
if you have a Title with v1024 and that nintendo release the same title with v2530 then the Wii will replace the file with the new one because it has a higher revision number.
It's what is used to update the console when you insert a disc, and what is used when you select "update from network" in the settings.
Nintendo always tried to prevent users from using pirated version of the IOS.
At first, the cIOS Title version was released with revision matching the version of the cIOS development.
slot 249 with cIOS rev14 had Title Version 14, etc.
But nintendo could just release a fake file to be installed in the slot249 with a higher title revision to automatically replace the cIOS rev14.

To prevent Nintendo from replacing slots, you can choose to change the title version to the maximum possible value (65535) so Nintendo will never be able to update it (as there's nothing higher).
The Title version as no other use than determine if there's an update and replace it with the new one.

selecting "21" or "21 008" (for cIOS 21 d2x v8) or "65535" are all exactly the same thing and will not affect your game compatibility.


Now about the cIOS development revision :
----------------------------------------------------------
developers updated the cIOS functions and fixed bugs, etc.
on every revision/version, it got increased :
cIOS rev14
cIOS rev15
cIOS rev16
cIOS rev17 <-- what Mighty channel is using
cIOS rev18
cIOS rev19
cIOS rev20
cIOS rev21

Starting from here, Waninkoko (the main cIOS developer) didn't release any more update of his cIOS. Someone else (Davebaol) did some modification of the cIOS rev21 and called it "cIOS rev21 d2x v1, etc."
d2x is in fact the continuation of the cIOS rev21, but instead of naming it "rev22" it became "rev21 d2x version xxx".
cIOS rev21 d2x v1
cIOS rev21 d2x v2
...
cIOS rev21 d2x v8 final
cIOS rev21 d2x v9 beta
cIOS rev21 d2x v10
cIOS rev21 d2x v10-alt

This above are all the different cIOS versions.
if you want "rev17" you need to install the cIOS rev17, you can't install d2x rev17, it does NOT exists. d2x is only "rev21" with "modified version" 1 to 10-alt.


ATTENTION :
if you use the cIOS 38 rev17 installer, it will automatically, and without letting you choose, REPLACE both slot249 AND slot250 with rev17.
You will have :
slot 249 : d2x rev17 base 38, Title revision 17
slot 250 : d2x rev17 base 38, Title revision 65535

Mighty Channel NEED rev17 to be in slot249, so if you currently have d2x v8 base56 installed in that slot, you should reinstall d2x base56 in a different slot (for example slot250, like I have on my console... look in my signature)
I recommend using Modmii instead if you want rev17 in slot249; that way it will install only in a single slot and will not delete your current slot250 (probably d2x base57)