The possible values of wbeuaspect are S, L, D, T, C, X, O. I guess L is for Label, D is for Description, S is probability for sitelink or statement, what about the others?
Topic on Talk:Wikibase/API
const SITELINK_USAGE = 'S';
const LABEL_USAGE = 'L';
const DESCRIPTION_USAGE = 'D';
const TITLE_USAGE = 'T';
const STATEMENT_USAGE = 'C';
const ALL_USAGE = 'X';
const OTHER_USAGE = 'O';
Thanks, Shirayuki ^_^