On mower tyres, and many ATV tyres, you'll often see 'NHS' or 'Not for Highway Service' next to or close to the size marking. This is principally a warning for the American market, to bring to the users attention that the tyre is not intended to be used on the road, at highway speeds.
Your replacement tyre does not need to carry the 'NHS' marking. If, for example you replace your 15x6.00-6 NHS tyre made by Carlisle with a similar tyre from Deli, that tyre will be marked 15x6.00-6 IMP. They are both intended for the same use.