The tolerance plus rating describes how high a given rating on the capacitor can go above its nominal rating and still be within specifications. As is the case with the tolerance minus rating, this has to be taken into account when selecting a capacitor for any application. It can be assumed that the capacitor will, under some conditions, reach the highest level of the tolerance plus rating, which still means that the capacitor is performing according to specifications.
Tolerance plus and tolerance minus are both typically combined into one figure, expressed as +/-. In a capacitor with a +/- of 20%, for example, the capacitor could be expected to function within 20% of its maximum or minimum value, and still be within the given operating specifications if it did so.
Aluminum capacitors are affordable, reliable solutions to many common electrical engineering needs. They do, however, have some distinctive drawbacks and inefficiencies that require that, when they are used in any application, these be taken into account and that they be accommodated for in the design of the device to avoid damage to the device overall or simple unpredictability in function.