The neutral axis reference identifies a P-line in the Structural Pointset. It is set by the NAREF command. The attribute is usually set to the name of the P-line, but may be set to the P-line’s number in the member list of the Pointset. For example:

MODEL will use as the neutral axis the first P-line in the Structural Pointset which has a PKEY value of NA, if any. Failing that, it will choose the first P-line with a PKEY value of NAXI, and failing that, it will choose the first P-line with a PKEY value of ZAXI. If there are no P-lines with a PKEY value of NA or NAXI or ZAXI, MODEL will assume that the neutral axis of the Component lies at the component origin and has a direction along the positive Y axis.