Once you have the correct signal type (usually the second digit in the model number), the main difference between Heidenhain glass scale model numbers is the resolution, reference mark configuration and physical size of the scale.
For example, an LS406 is the small size with high resolution- LS303 is the same body as the LS406 but with lower resolution. Generally speaking the LS3XX,4XX,8XX,9XX are the smaller bodied scales, the LS1XX are larger body. Some scales have removable markers that allow you to position the reference mark according to your preference. The ND960 has parameters in it's software which allow you to match the DRO to the scale resolution, including custom display characteristics, like how many zeroes you want on the right side of the decimal point.
Have to study carefully as Heidenhain has a large number of scale options- different grating materials, resolution, signal format, reference marks, body size, connector types, mounting types, cables...
Depending on the type of work you do, LS303 scales ought to be great on your FP2, the ND960 can be set for them to display .0002 resolution. If you want finer than that, opt for the LS403 or LS406.