Comparing MO Labelling Schemes: LHS vs. RHS

[etotheipi]

Two different labelling schemes for MOs are shown on the left and right of this diagram:

I understand the LHS naming scheme, since the MOs are simply labelled according to the AOs from which they are derived (along with $\sigma$/$\pi$ and $u$/$g$ for the MO).

I don't understand the numbering scheme on the RHS. The book says that 'orbitals of the same symmetry are distinguished from one another by numbering them 1,2,...'. What does this mean? Thanks.

 

[DrDu]

There is the first sigma_g orbital, the second sigma_g orbital, the first sigma_u orbital, the second sigma_u orbital ... The symmetry is specified here by sigma/pi and the u/g label.

 

[etotheipi]

There is the first sigma_g orbital, the second sigma_g orbital, the first sigma_u orbital, the second sigma_u orbital ... The symmetry is specified here by sigma/pi and the u/g label.

Ah, I see. Thanks!