Some NPO (Non-POlarized) capacitors may have a stripe on their package. Since this can’t be a polarization indication, you might wonder what it means.
A capacitor is made of two conductive elements separated by a non conductive element. In cylindric capacitors (both radial and axial), there is a central core for one conductive element, and an outer shell that makes the second, with some non-conductive substrate in the middle.
The mark on an NPO point to the outer shell. This can be interesting if you need an end of your cap to be grounded. Grounding the outer shell provides isolation from EMI and prevents noise getting in the capacitor.
Check out the layout design tips and tricks that can take advantage of this.
Source: Hack a Day.