Your sewing machine manual should tell you the correct needle class (sometimes called system) for your machine. Needle classes define the design of the needle (flat shank, round shank, scarf, etc). Each needle manufacturer further confuses us by using their own numbering system. An Organ HAx1 is a standard home sewing machine needle class, same as a 15x1 or 130/705H. An HLx5 is a stronger needle, with a flat shank, like a standard home sewing machine needle.
Then there are needle sizes. These are related to the thickness of the fabric and thickness of thread on which they will sew well. 75/11, 80/12, 90/14, etc. Thin or sheer fabrics get lower numbers, thick fabrics get higher numbers. Needles also come with different points—sharp, ball point, quilting, etc.