Mark of...

[Seighart]

Yeah, i don't care for spoilers...since im in this section, but can someone explain to me what this actually is?

[Golden Yak]

Never heard of it. What's the context?

[Sora2016]

Oh okay this is in the spoiler section good :P Though the title may still be spoiler-ish.

The extent I think we know so far is that it's the X on Sora's new clothes, and therefore Eraqus, Terra, Aqua, and Ven also have one as well. Apparently the mark allowed Xehanort to track Sora, hence how he always appears in the exact location Sora is throughout the game.

[VLR.XIII]

Sora2016 is pretty much spot on.

X's on clothing were used as a tracker by Xehanort throughout the series.

[Crystal]

Did Riku having X mark on his cloth?
And when does it appear on Sora's cloth? xD

[Hillboy]

Wut.... Minddiddly.... 0_0, his clothes have been a trap from the start :/

[Memory Master]

Its pretty obvious how Xehanort was able to get Sora to wear clothing with X on it.

Sora's mom went to Destiny Islands Wal-Mart to buy clothes for Sora. Xemnas disguised as a Wal-Mart employee comes up to her makes some suggestions.

Xemnas: Excuse me ma'am. Clothes with X's in the design are all the rage right now. I'm sure your son would love to have his entire wardrobe consist of them!

Sora's Mom: How do you know I have a son?

Xemnas: Uh I uh....well why else would you be shopping in the young men's section?

Sora's Mom: Well I don't know, his entire wardrobe consisting of this stuff?

Xemnas: They're all 80% off! No scratch that, they're free! Just get them and get out!

Sora's Mom: Okay okay...sheesh!

Xemnas watches Sora's mom leave with the X clothes and smirks.

Xemnas: I placed a fragment of my heart inside of her.

Saix appears next to Xemnas: I always knew you wanted to be inside of Sora's mom, lord Xemnas.

Xemnas: What? She's a got a nice ass. Besides when I become king of the universe I'm going to need a Queen. (Spoiler for KH3)

[Shawty]

^ Yup, that must be how it happened XD

[Seighart]

Hm..thats strange. I wonder how exactly it gets there...is it from the keyblade?

[Stars]

Did Riku having X mark on his cloth?
And when does it appear on Sora's cloth? xD

It appear on Sora's cloth when he reached Traverse Town and start his journey, if i'm not mistaken
And the X mark should have been placed on Sora's cloth when he is on the way to TT? Probably the brown cloaked Ansem? xD