apns-conf.xml formatting?

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Does anyone happen to know of a referrence for correct formatting of entries in the apns-conf.xml file?

I'm trying to put together a couple of custom entries for Canadian carriers but I want to make sure they are formatted correctly so as not to step over existing entries.

Speficically, I'm curious what the acceptable options are for type=. I know it can be blank, default, or mms but is there also an option for vpn for some carriers that have a different APN for vpn connections or what is the correct entry if you want to have multiple APN options for a single mcc,mnc combination.

I've tried googling and checked at developer.android.com but can't seem to find any referrence to correct syntax for that file.