The difference between the two is whether the chemical is identified on a GreenScreen List Translator (GSLT).
LT-UNK stands for List Translator Benchmark Unknown. These are substances for which the hazard lists give us some information towards determining a GreenScreen score, but the listing is not sufficient to determine a Benchmark 1 rating.
NoGS stands for No GreenScreen score. This means that the substance is not identified on any GSLT list, nor does it have a public GreenScreen assessment.