In the Omnimech Rules and Construction post there is an error. This part is inaccurate:
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A player cannot reach the Master level of the 'Mech Tree of a configuration without first completing the Elite level for three configurations of the same chassis.
This is in the same manner as standard 'Mech variants.
Additional Clan 'Mech Rules
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A lot of people are confused by this, so I'm not surprised that the writer is as well. You will unlock the MASTER efficiency if you have completed ELITES on ANY 3 CHASSIS IN THE SAME WEIGHT CLASS. For example, JoeMechDude will unlock the MASTER efficiency on his VTR-9B once he completes all ELITE efficiencies because he previously completed all ELITE efficiencies on his AWS-9M and his AS7-D.