Ainadaliel,
Soil- the natural soil or dirt that is outside in the garden naturally, containing leaves, worms, whatever, but your exsisting outside dirt.
Soil-less potting mix- Used for POTS only, made specifically for pot use, containing larger fragments for the plants to allow their roots to "breathe" and drain well. Most soil-less potting mix also contains some kind of fertilizer because the plants can't get the natural fertilization in the soil outside, like leaves, worm castings, ect.
Where are you starting your plants?
Most people start them inside in the winter, which would mean you aren't planting them in the ground.
So, if you start babies inside, you want a pot with soil-less potting mix.
When they get big, and you want to plant them outside in your garden, then they go in your soil(which is your dirt outside in your garden bed) mixed with compost.
I don't know what fungus has to do with anything, if you get fungus on them inside, the drainage isn't good and they aren't getting enough fresh air.

Thanks! This makes a lot of sense now. :)

Pots need a soil less potting mix for drainage because regular garden soil will tend to become water logged in a pot. I have used the compost I make for a potting soil for years with no such problem.

You may find the information at the link below of interest. It explains why soil/water/air relationships are so much more critical in conventional container culture than in your gardens/beds, and why soils comprised primarily of small particles (topsoil/peat/compost/coir) are more difficult to grow in and provide a much narrower margin for error than soils the primary fraction of which are comprised of larger particulates.

Al

Here is a link that might be useful: More about container media