Usually the first line of a filters file is simply [Adblock]. However, you might have noticed that recent versions of Adblock Plus sometimes put a different text instead. This is done when you have filters in your list that use advanced filter syntax only supported by newer versions of Adblock Plus but not original Adblock. One example would be:
(Adblock Plus 0.6.1.2 or higher required) [Adblock]
This is simply a comment. Adblock (and Adblock Plus for that reason) will ignore anything before the actual mark. The required Adblock Plus version is not enforced because Adblock Plus 0.6.1.2 didn't support it. However, if you use even newer filter syntax, you might get something like:
[Adblock Plus 0.7.1]
This type of header is supported starting with Adblock Plus 0.7.1. Older Adblock Plus versions and Adblock cannot open files starting with this header. As to the current versions, they will check the version number in the header and compare it with their own version number. If the file happens to require a newer Adblock Plus, the user will be given a message on import asking him to upgrade. Subscriptions will still load files meant for newer Adblock Plus versions but display a warning in the preferences dialog.
Finally, if you want to require Adblock Plus but don't want to specify the version number you can start the file with [Adblock Plus]. Of course this file will only be accepted by Adblock Plus 0.7.1 or higher again.