Information presented below this line is outdated syntax or information used for Custom Stuff 1. It will not work with Custom Stuff 2.
Redstoneonly is an attribute that is exclusive to door-, trapdoor-, and fencegate-type blocks. By default, this attribute is set to false, and is thus not normally included at all. When set to true, the affected block's position can only be opened by a redstone current. That is to say, you require a redstone mechanism to open them, and they're no longer able to be opened by hand.
The attribute can be changed to 'true' by adding the following line to your .block file:
In the vanilla game, the Iron Door displays the behavior of the redstoneonly attribute being set to true.