The ICON / ICON + filters are designed to prevent unwanted dust and debris from entering the CPAP machine. it is important to change these on a regular basis to maintain the CPAP in good working order. The filter is a graduated filter to optimise the lifespan of the filter and has different thickness grading throughout the filter. The black line is an indicator on which side goes into the machine and which is on the outer part of the filter holder with the black mark orientated on the inside. COMPATIBILITY: Icon+ Novo CPAP, Icon+ Premo CPAP, Icon+ Auto APAP, Icon Novo CPAP, Icon Premo CPAP, Icon Auto APAP *Device not included