There are two types of spells; static & dynamic spells. Magical exhaustion has to do with how much energy you can take.

Static Spells & Dynamic Spells

For static spells, there is no prolonged concentration. The concentration remains only as long or as little as it takes to use the spell & to acknowledge the result. An example of this type is when using the Knockback Jinx. Dynamic spells will usually have longer effects with constant willpower & concentration aimed at the objective. It can also have an outcome be altered while casting. An example of this type is the Patronus Charm. A few spells can be both categories.

Magical Exhaustion

Static spells will use your magical energy to cast, while dynamic spells will use your magical energy continuously until the spell vanishes. If you were to cast a spell, even static spells, over & over again, you will ultimately start to tire out. If you channel too much magical energy at once or cast one too many spells, you may feel sick to your stomach, find it harder to breathe, or feel exhausted. Magical exhaustion also has an indication of being vulnerable to unanticipated magic.

Fun Facts
⊱ For static spells, the concentration doesn't go farther than the amount needed (the incantation & wand motion).
⊱ For the Patronus Charm, you will be required to aim the spell towards Dementors to strike them.
⊱ If you lose track of your concentration in dynamic spells, even for a few seconds, the spell will stop working.
⊱The Fire-Making Charm can light a candle (static spell), & can also create a stream of fire (dynamic spell).