[Grammar] Question for distinguish between attributive clause and appositive clause

[xobli]

Hi, to whom is may concern

It is difficult for me to identify difference betweem these two kind of clauses. I think they both can go behind "that", and sometimes look the same to me, especially when a that clause is very long.

Bo

 

[Tdol]

I honestly wouldn't worry about it; 99% of native speakers and learners wouldn't know the difference, which isn't always clear anyway.

 

[MikeNewYork]

The opening phrase should be "to whom it may concern".