What do "local", "non-local", and "external" features mean regarding protein secondary structures?

Question

[4fzcgueyp5]

Take this paper as an example.

The paper repeatedly talks about local, non-local, and external features. Some of them are distance, geometry, angle, hydrogen bond count, etc.

What do "local", "non-local", and "external" features mean regarding protein secondary structures?

[GenoMax]

Looking at the paper where do you see "external" features mentioned? I only see local and non-local features in text. Word "external" does not seem to be there in the paper.

[4fzcgueyp5]

http://www.diva-portal.org/smash/get/diva2:1041336/FULLTEXT01.pdf

https://www.researchgate.net/figure/External-feature-alignment-using-protein-secondary-structure-generally-improves-accuracy_fig3_23685272

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2647288/