What do these dimensions mean?

[satprof]

I thought that I'd been around the DIY block a fair number of times, but this measurement has me stumped, so advice from real experts would be very gratefully received.

The worktop cut-out for an induction hob is specified as being 560mm +4 +1 x 490mm +4 +1.
What's with the +4 +1? I'm guessing it has to do with tolerances, but most specs simply specify, say, 560mm x 490mm.

Thanks in advance.

 

[Notch7]

Its a typo,

It is a tolerance:

I would guess at +4 -1

 

[bobasd]

lie the hob upside down in the correct position on top of the worktop.
mark an outline around the edge.
remove the hob.
mark another outline approx 8/10mm to 15mm or so according to width of the hob edge fin or mfr's instructions.
the inside outline is the cutout.

 

[foxhole]

Tolerance , you can cut to dimension up to 4mm larger or 1 mm smaller.