NS 44 with fine particle retention is thinner than NS 42. The flow rate of NS 44 is twice of NS 42.It’s a filter paper for critical gravimetric analysis and metal determination of very fine particles such as barium and lead sulfates, calcium oxalate and calcium fluoride.
https://www.newstarsciences.com/chemical-analysis-filter-paper/grade-44-same-to-whatman.html