Tony M Oct 3, 2018:

@ Asker Usually, 'positive tolerance' means that [some value] may be hgiher than its [nominal value], but in no case lower — or at any rate, by a smaller margin; like for example 200 V +20% —5%; thus it generally expresses an asymmetrical tolerance, whereas they are very often symmetrical: 230 V ± 10%.
However, as you say this is written by a non-native, it is possible that this is not quite how it is being used here! Possibly your wider context will make it clearer...
I think Gilou's suggestion would be correct, on that assumption.