Definition of Full-scale error
An error expressed as a percentage of full scale means that any measurement made will fall between the limits given by that quantity.

For example, an analog voltmeter with a range of zero to 100 volts specified as having an accuracy of +/-1% of full scale can be off by plus or minus 1 volt when there's between zero and 100 volts across the meter. 

So, with the meter reading 1 volt, the actual input could be anything between zero and two volts, and with the meter reading 100 volts the actual input could be anywhere between 99 and 101 volts.