[2022] Requirements to apply elemental ailments?

By default:

• Fire Damage from hits can Ignite
• Cold Damage from hits can Freeze
• Cold Damage from hits can Chill
• Lightning Damage from hits can Shock
• Hits always Chill
• Critical Strikes always Ignite
• Critical Strikes always Freeze
• Critical Strikes always Shock

The "can" in those first four means that if the hit tries to apply that ailment, damage of that type is counted for determining how strong the ailment will be (a hit can't inflict an ailment at all if it deals no damage of a type that can inflict it).

The "always" in the second four means the same as having 100% chance to inflict the ailment - the hit will always check how much damage it does of types that can inflict that ailment, and attempt to inflict it (but will fail if it dealt no damage of approriate types, or not enough damage to the point the ailment would be insignificant and is skipped).

You need both things to match up in order to actaully inflict an ailment - The hit needs to try to inflict the ailment, and needs to have dealt some damage of a type that can inflict it. Various stats can add chances for hits to inflict the ailments (redundant on crits, but matters for regular hits), or make different damage types able to inflict them.

In general, how strong an ailment is depends on how much damage of the apporpriate type(s) you dealt compared to the enemy's life, and that needs to be enough to meet a minimum magnitude - ailments that are too weak will not happen. Freeze instead has a fixed base magnitude but scales duration, and that needs to be at least 300ms to be inflicted.

There are no restrictions on which ailments can be on the same enemy.