There are two main types of starter,electronic and conventional starters.

The advantages of an electronic starters are, they "soft start " which extends the life of the tubes, reduced flickering, will cut out on tube failure they simply stop the tube rather then let it strobe.

Electronic starters will work with any length tube.

Ordinary starters are specific to certain lengths.

For 2 ft tubes in multi tube fittings you need a series starter FS2 as the lamps share a ballast.

Up to 5 ft an FSUstarter is required.

6 ft 70 watt tubes need an S16 starter.

Above 6 ft you need an FS125.

These are also known as:

starter motor chokes cans ignitors