_priority priority of adding the code, use the constants PRI_HIGH, PRI_NORMAL or PRI_LOW (don't forget to add OutputSystem::). The changes the order in which the codes are added.
_ms-expression if you want to add something only for a particular IE, you can use this to add a '<!--[(expression)]> ....<![endif]-->' around the tag.
Add a callback which will be called on the html. If true is returned than the code will be enabled.