As a learner, it can only at any time beneficial for you to distinguish /ʊ/ and /u:/, not /ʊ/ and /u/, and you'll try this by length.I do think the dialogue about /x/ and /χ/ is superfluous. I checked several WIKI-pages and whilst the one particular suggests that /x/ is Completely wrong the other says /x/ is right.Broaden your vocabulary two met