Thanks, @Alberto.Utrera, for answering @Achilleas.Kazerakis questions!
RNA refers to the rotor-nacelle assembly.
The OpenFAST linearizations do not show the mass matrix directly, but this matrix can often be inferred from other matrices generated through the linearization process. A similar question was asked and answered in the following forum topic: http://forums.nrel.gov/t/openfast-2nd-order-linearization/2249/2 . To get only the RNA mass matrix, set the tower and platform masses to zero.
Best regards,