Wen alloys featurizer documentation

I am using the Wen Alloys featurizer from matminer.featurizers.composition.alloys
Everything works fine, but it generates a feature named, ‘APE mean’. Unfortunately, I have been unable to find out what APE refers to - even from the paper that is referred to (Cheng Wen et al. Acta Mater. 2019). It would be great if someone can clarify this.