请教provided/if 区别

请教provided/if 区别

LDOCE
PROVIDED
used to say that something will only be possible if something else happens or is done
synonym providing
He can come with us, provided he pays for his own meals.
There's no annual fee provided that you use the credit card at least six times a year.

Cambridge Learner's Dictionary
provided (that)
only if
He's welcome to come along, provided that he behaves himself.

剑桥国际英语词典（CIDE）的解释是：
if, or only if
Provided (that) there are no objections, we will begin with item three on the agenda.
He says he'll go parachuting provided/providing (that) you do too.

SOED
provided
On the condition, supposition, or understanding that, or that; it being stipulated or arranged that or that.
Allowed to talk..provided that they did not raise their voices.
M. Keane Jane did not mind expense provided she got what she wanted.
（注：He
was released on bail on condition that he did not go within half a mile
of his mother’s address. On the condition表达的明显是必要条件。）

总结：虽然有些词典也认为其有IF的含义，但大多数还是认可且只认可作only if解。而only if的意思是“只有”，表达的是必要条件（上面所有的例子我们当作必要条件来理解都不会出现荒谬的意思）。相反IF表达的是充分条件。这两个词的区别是不是主要在这里？
至于OG 上说的：B uses the pretentious and illogical word provided for if in a conditional clause after a negative idea (would fail).
是不是说这里用必要条件不合适？
第二个例子前面就是否定的意思。

在句中当“假如、倘若”用时毫无差别； if 比较平易近人，老妪能解。provided 经常在商业文书、法律文件中出现，比较正式。