What's the definition of the paper_iCluster.Group variable

[beerzyp]

LUAD clinical dataset contains a variable paper_iCluster.Group. What does it mean, I can't find the definition anywhere on the TCGA metadata documentation.

[beerzyp]

This variable takes integer values from [1, 6] and has missing values from some patients, I'm interested in knowing how it was classified and why is it present only in the LUAD dataset, as it's missing for LUSC patients.

My web search found some results for iCluster referencing Hoshida et Al and PanCancerAtlas, however I still don't have the answer for how these clusters were created and why they don't show up in TCGA documentation. I'm also interested in why they are present in LUAD but not in LUSC datasets.

Edit: Both papers referenced about talk about clustering types of cancer only, the iCluster.group variable relates to cancer sub-types which is what I'm looking for. I also found this paper which does explain a methodology to cluster LUAD and LUSC patients however I'm almost certain it doesn't relate to the variable in the TCGA dataset.

Thanks,