which meta-dimensions are valid for which major modes.
If t, all meta-dimensions are allowed in all major modes.
Otherwise, it is an alist mapping modes to sublists describing the meta-dimensions allowed in that mode.
Each sublist should begin with t, to indicate that only the meta-dimensions listed are to be allowed, or nil, to indicate that all meta-dimensions except those listed are allowed.
The rest of the sublist is the meta-dimensions allowed or blocked for that mode.
The head of the node may also be a list of major modes for which this rule applies.
A sublist for a major mode t gives the defaults.