The best way to use quiconque is in the meaning of whoever. However, sometimes quiconque can be substituted by n' importe qui or tout le monde. And the the next question is, if n' importe qui is anybody and quiconque can be anybody, where is the difference?
Quiconque as well as n' importe qui and tout le monde can be used, if each member of a group can be initiator of the action, but NOT all together.
Each and everyone can be initiator or can be affected by an action.
Quiconque peut aller à la plage.
Anybody can go to the beach.
Tout le monde peut aller à la plage.
All can go to the beach.
N' importe qui peut aller à la plage.
Anyone can go to the beach.
Je le ferais pour quiconque.
I would do it for anybody.
Je le ferais pour tout le monde.
I would do it for everybody.
Je le ferais pour n' importe qui.
I would do it for anybody.
Nous ne permettons pas à quiconque de se
mêler de nos affaires.
We do not permit anyone to meddle
in our affairs.
Nous ne permettons pas à tout le monde de se
mêler de nos affaires.
We do not permit anyone to meddle
in our affairs.
Nous ne permettons pas à n' importe qui de se
mêler de nos affaires.
We do not permit anyone to meddle in our affairs.
And if quiconque can be substituted by tout le monde (= everybody) and chacun is everybody, where again is the difference. Chacun can never substitute quiconque, because chacun is equivalent to each and everybody, meaning that each member of the group (and not any member of the group) can be part of the action.
Now, you might ask, when quiconque, n' importe qui and tout le monde cannot be substituted? Whenever the initiator of the action or the one affected by the action is unknown.
Examples
Quiconque a dit ça, est un imbécile.
Whoever said this, is an idiot.
NOT: Anybody who said this ...
NOT: All who said this ...
and NOT: Everybody who said this ...
because we do not know the one who said this...
BUT:Celui qui a dit ça, est un imbécile.
The one who said this, is an idiot.