Player::addAttachment();

Can someone please give me an in-depth description of what a plugin attachment does exactly and how to use it in a permissions plugin? I've tried looking at PurePerms, but I'm having a hard time making sense of exactly how they work.

Not Really Sure.

public function getAttachment(){
return $this->attachment;
}
from:
http://docs.pocketmine.net/d0/d04/_permission_attachment_info_8php_source.html
http://docs.pocketmine.net/d5/d9f/classpocketmine_1_1permission_1_1_permission_attachment_info.html

I have less experience with PHP than you so I don't know if this helps.

Nope. I want to know what exactly the purpose of a player attachment is and how to use it.

PHP:

$att= $permissible->addAttachnent($plugin); # adds permission attachment to the permissible; I'm not sure if it gets auto removed if the plugin is disabled.
$att->setPermission($name, $boolean); # set permission $name to $boolean

$permissible->removeAttachment($att); # run this to remove your attachment and garbage it, esp. when a player quits.

That's not exactly an in-depth explanation, it doesn't even explain what an attachment actually does.

Are players permissible?

Yes. Read the docs. The inheritance diagram shows that.