scoping - What is the purpose of Internal`LocalizedBlock?

I have come across the (internal) use of the function:

Internal`LocalizedBlock

I am trying to determine its purpose. It seem to behave like Internal`InheritedBlock except that a starting value (e.g. {x = 3}) cannot be set.

x = "global"; f[] := x

Internal`LocalizedBlock[{x}, {x, x = 7, f[], Hold[x]}]
x

Internal`InheritedBlock[{x}, {x, x = 7, f[], Hold[x]}]
x

{"global", 7, 7, Hold[x]}

"global"

{"global", 7, 7, Hold[x]}

"global"

What purpose does this function serve? Why would it be used in place of InheritedBlock?

Answer

Internal`LocalizedBlock behaves the same as Block, but it can localize non-Symbols (e.g. f[1], Subscript[x, 0], etc.).

For example,

Internal`LocalizedBlock[{Subscript[x, 0]}, Subscript[x, 0] = 1]
(* 1 *)

Compare this to

Block[{Subscript[x, 0]}, Subscript[x, 0] = 1]
(* During evaluation of In[79]:= Block::lvsym: Local variable specification {Subscript[x, 0]} contains Subscript[x, 0], which is not a symbol or an assignment to a symbol. >> *)
(* Block[{Subscript[x, 0]}, Subscript[x, 0] = 1] *)

It's also worth noting that one cannot assign values in the first argument of Internal`LocalizedBlock

Comments

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