Yes, there can be some confusion around OSGB co-ordinates.

Eg the summit of Ben Macdui is at WGS84: 57.070419, -3.669233

Looking at an OS Map, using traditional 6-fig grid ref it is at NN988989

This grid ref has a 100m x 100m resolution, which is useful enough for paper maps.

Some co-ordinate conversion tools will convert the WGS84 to OSGB as 298891, 798948

The leading 2 and 7 define the NN 100k grid, per the picture above.

The additional 2 digits at the end (xxxx91, xxxx89) increase the resolution down to 1m x 1m, which is in the same order of magnitude as typical GPS accuracy ( say 3 to 5 m with a good sky view ).

The confusion with OSGB co-ords lies in whether the initial digit is the 100k square designator, or if it's the start of a higher-resolution co-ordinate within the 100k square.

My basic rule-of -thumb is:

If there are 6 digits, it's a traditional 100m x 100m resolution, and the 100k designator is required.

If there are 8 digits, it's *probably* a 10m x 10m resolution, 100k designator required.

If there are 10 digits, it's *probably* a 1m x 1m resolution, 100k designator required.

If there 12 digits, it's a 1m x 1m resolution, and the leading digits fully qualify the 100km square.