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Why is it that some units need to be included in the units block but other don't?
MH
ex. why does "(cm2) = (centimeter squared)" not have to be placed in the units block but it is used in the PARAMETER and ASSIGNED blocks? (see ex. 8.4.4.1 in The NEURON Book)

Also, what is "(mol) = 1" in the units block in that same example?

Why is it that some units need to be included in the units block but other don't?

To gain a proper understanding of the role of units declarations in NEURON, please read
the following in The NEURON Book:
9.2.2 Variable declaration blocks on pages 211-212
9.5.2 The UNITS block on page 222

ex. why does "(cm2) = (centimeter squared)" not have to be placed in the units block but it is used in the PARAMETER and ASSIGNED blocks? (see ex. 8.4.4.1 in The NEURON Book)

Does it? I haven't seen the statement
(cm2) = (centimeter squared)
in any NMODL code anywhere, let alone in The NEURON Book.

what is "(mol) = 1" in the units block in that same example?

See if reading 9.5.2 The UNITS block on page 222 of The NEURON Book answers
the question.