I have not idea, but does it relate to grading or size of particle retention? http://www.labfilter.com/article.asp?id=196&title=About_different_glass_filter_grades
“but its impinger has a porous glass filter plate of porosity 1 (ø of pores 100- 160μm) at its end.” At page 112 of http://e-collection.ethbib.ethz.ch/eserv/eth:24354/eth-24354-02.pdf