Can be true only on the outermost members on the hierarchy (row or column) of the Tablix.
Members with FixedData=True must be contiguous with all other FixedData members on the hierarchy.
Not allowed to be true if the Tablix has headers on the opposite hierarchy (FixedRowHeaders or FixedColumnHeaders).
Not allowed to be true on the leftmost column member if GroupsBeforeRowHeaders is set.
Not allowed to be true on row members unless set on the first row member.
Not allowed to be true if a corresponding body cell is part of a span and the FixedData for a peer TablixMember corresponding to another cell in the spanned area is false.