Experience (XP)

When killing zombies, they drop experience in the form of green and blue crystals.
When selecting crystals, the upper scale is filled in as a percentage.

At the initial stage, you need to pick up 10 green crystals to fill the scale and get the opportunity to choose one skill.

The blue crystal gives 3 times more experience than the green one.

Elite zombies drop a purple capsule, allowing you to select an elite skill skill (but not required).

The boss drops a red-purple capsule, which gives a book (volume) of skills to choose from.

In the game there are various items and bonuses that increase the experience gained in battle.

Here they are:

If you take an XP assistant robot with you into battle, then now 1 crystal at the beginning will give not 10, but 11% of experience in the upper scale.

Even if you collect all the items that increase experience gain, this does not mean that you will receive more skills per battle. You will receive them sooner.