By that I mean if you are getting along fine with a 38 guage triangular needle, you may find that you can in fact work a lot more efficiently with a 38 guage STAR needle. Or , if you are dealing with strong , coarse fibres, say Welsh Mountain Wool, a 36 tri needle will be far more effective.Romney wool will be the same (it's coarse too).
The gauge of the needle refers to it's diameter. Needle gauges range from 32 up to 42.Then you get the CROWN needle, with similar guages but with only one notch on each face(mostly used for inserting hair or eyebrows on dolls), regular needles have two on each face (edge) That's six on a tri and eight on a star.
The higher the gauge --the finer the needle. When you start a project, it is common to choose a needle that is thicker in diameter as it felts your fibre faster. Higher gauge needles are used for adding details to your piece and smoothing the surface
3D or Flat Felt
There are reasons , which are gained only from experience, why certain needles are preferred when making either a 3d piece such as a doll or a flat felted item, such as a picture
When making 3d pieces, the aim is to start coarse, to build up up quickly, finish fine,get a really smooth appearance to the surface of the finished item, so a super fine needle is the order of the day. This can be as fine as a forty two guage or a forty gauge , which ever you prefer, my personal preference is both!! Start the finishing process with a forty and finally a forty two , to make sure there are as few needle marks as possible.
I know to the beginner, that this may all sound extremely confusing, It's really not, just get yourself a selection to start you off, have a crack at it and 'feel your way round' !!
You will start to find, that one feels better at certain jobs than another.