A different error estimator is based on the difference between the maximum and minimum first (=maximal) principal stress at the integration points in the elements belonging to one and the same node . It is triggered by selecting ERR underneath the *EL FILE keyword card.
A node usually belongs to several elements. The stresses are available (and most accurate) at the integration points of these elements. If the largest difference between the first principal stresses at all integration points within an element is small, the stresses vary little across the element and the element size is deemed adequate to yield an accurate stress prediction. Both the absolute value of the largest difference and the relative value are calculated. The relative value is the absolute value divided by the absolute value of the largest first principal stress within the element.
To obtain values at the nodes the maximum is taken across all elements belonging to the node. In a strict sense this is not an error estimator, it is just a measure for the variation of the stress across all elements belonging to the node.
For heat transfer a similar error estimator was coded for the heat flux. It is triggered by selecting HER underneath the *EL FILE keyword card. It represents the variation of the size of the heat flux vector across all elements belonging to one and the same node.