PIMD: Extra credits

How do you get Extra Credits?
You can get extra credits by completing jobs. Every so often, jobs will give you 1 extra credit. Extra credits are also rewarded when you achieve certain goals throughout the game.

What are extra credits used for?
Extra credits can be exchanged for cash or regeneration. Some items can only be purchased with extra credits. Items bought with extra credits cannot be sold for cash.

Source

PIMD: Jobs

Why does the difficulty of a job keep changing?
The difficulty of a job is based on your current strength. If you lose you strength due to jobs or battles, the difficulty of the jobs will increase. If you regenerate your strength, the difficulty of the job will decrease.

What do you get from jobs?
Jobs give you cash and on rare instances Extra Credits and items.Source:

NB: Continue doing jobs even if you're done with it because they will still give you the chance of having free Extra Credits or Dr's Notes. Repetition of doing Jobs will give you more Credits after 24hrs.Based on my experience.