As you advance, your Bridge Operator (and every crew member too), will level up when you fill the XP bar, giving you acess to better and better ships. At level 12, you will be able to pick a nationality. There are 4 nationalities, the German, the British, the United States, and the Japanese. When you continue upwards in levels the amout of EXP per level can increase a few thousand each level. The highest practical level (if not the maximum) is 120, where you unlock the top-of-the-line ships.