The shop page lists what is published, with a filter column beside it. Category pages use the same layout, limited to that category.
Category is the product category. Right now the products sit in Lab Consumables. Lab Organization, Pipetting Accessories, and Benchtop Tools are on the menu, and they use the same catalog. If a category has no products, the page says so. It does not borrow items from another category to fill the grid.
Inside a category that has products, or on the shop page, you can narrow by membrane (Nylon, PES, PTFE, MCE, PVDF, BT), diameter (13 mm, 25 mm, 33 mm), pore size (0.22, 0.45, 0.8, 1.2, 2.0 µm), and pack size (5, 10, 25, 50, 100 pieces). Price is a minimum and a maximum. In stock hides what is not available.
Choosing two membranes means either membrane. Choosing a membrane and a pack size means both. Clear all returns you to the unfiltered list. On a category page, Clear all stays on that category.
If a combination returns nothing, that combination is not in the catalog. It is not a hidden product. Write if you were looking for a size the list does not have, and include the four lines from how to read a title so we can see what you meant.