The 3T, 4T, and 10T are spawn-master bags with a 0.2 micron filter patch — sized for grain (3T smallest, 10T largest, 4T the most common middle ground). The XLS-A is a sawdust / bulk substrate bag with a coarser 0.5 micron filter sized for hardwood-pellet bulk substrate runs where the substrate breathes more aggressively during colonization. Match the bag to the medium you're filling.

The filter patch on a Unicorn bag lets gas (CO₂ out, O₂ in) pass through while blocking airborne contaminant spores. The micron number is the pore size: 0.2 micron blocks essentially all bacterial and mold spores, used on grain bags where contamination risk is highest. 0.5 micron allows more aggressive gas exchange for bulk substrate where the substrate is producing more CO₂ during heavy colonization. Both are sterilization-safe; the difference is breathability vs filtration tightness.

Three reasons. Custom formulations — mixing your own substrate ratios or grain blends. Scale economics — once you're running enough volume, sterilizing your own bags is cheaper per pound than buying pre-sterilized. Specialty species — some species do best with substrate recipes we don't carry pre-sterilized. The trade-off is needing your own pressure sterilizer or steamer at the scale where this makes sense.

Self-adhesive ports that stick onto any sterile vessel — a canning jar lid, a bag without a built-in port, a culture flask. Peel the backing, press onto a clean surface, and the port self-heals between needle injections. Useful when you're working with bags or vessels that didn't ship with ports built in (most XLS-A bulk substrate bag prep, for example), or when scaling a workflow where the right pre-fab vessel isn't available.