This may seem obvious if the pattern says using 9mm needles knit 14 rows over 10 stiches should = 10 cm then do they mean 10 cm both lengthwise and widthwise.
Ideally you shoud knit your swatch a bit bigger than 10 cm/4 inches because stitches tend to be a bit wonky at the edges, I’d say go for about 15 cm/6 inches square. Measure your stitches in the middle of your swatch. Your stitches per cm/inch (width) is usually more important than your rows per cm/inch because many patterns will just say “knit until piece meaures ____ cm/inches from cast-on edge (or other reference point)”
If you find your swatch has more stitches than the pattern requires, move up a needle size, if you have too few stitches, try a smaller needle.
For the quick and concise response. That’ s really helpful - I’m sure many knitters struggle with this and I’ve made enough things recently that are off because I haven’t checked this before I start. I’m learning to be more careful of the bat. Thanks again.