Что касается удержанного налога, то он уже указан.

Что касается is a fixed expression meaning as for, regarding, or with regard to.

In this sentence, it introduces the topic:

Что касается удержанного налога, то он уже указан.
= As for the withheld tax, it has already been indicated/listed.

It is very common in formal or semi-formal Russian when the speaker wants to shift to a new topic or comment specifically on one item.

Because что касается normally requires the genitive.

So:

• налог → nominative singular
• удержанного налога → genitive singular

The same happens with the adjective/participle modifying it:

• удержанный налог → nominative
• удержанного налога → genitive

So after что касается, you should expect the thing being discussed to appear in the genitive.

Удержанного is the full form of a past passive participle from удержать.

Here it means withheld.

So:

• удержать = to withhold, deduct, retain
• удержанный налог = withheld tax

In this sentence it appears in the genitive singular masculine because it modifies налога, which is also genitive singular masculine.

So the phrase literally contains something like of the withheld tax after что касается.

The то helps mark the main clause after the introductory topic phrase.

Structure:

• Что касается X, то Y.
• As for X, Y.

This то is very common in Russian with this construction, especially in more careful or formal speech. It makes the sentence feel balanced and clear.

You may sometimes see что касается without то, but with this exact structure, то is very natural.

Yes, налога already identifies the topic, but it is inside the introductory phrase что касается удержанного налога. In the main clause, Russian often uses a resumptive pronoun:

• Что касается удержанного налога, то он уже указан.

This is similar to English topic structures like:

• As for the tax, it has already been listed.

The pronoun он refers back to налог and serves as the subject of указан.

Without он, this exact sentence would sound incomplete, because указан needs a subject in the main clause.

Указан is the short-form passive participle of указать.

In meaning, it is often translated like an adjective or passive verb:

• specified
• indicated
• listed
• stated

Short forms in Russian are commonly used in predicate position, especially after an implied is/was:

• Он указан. = It is indicated/listed.

So here:

• он уже указан = it has already been indicated/listed

Because it agrees with он, which refers back to налог.

• налог is masculine singular
• therefore он is masculine singular
• therefore the short-form participle is указан

Compare:

• сумма указана — feminine
• число указано — neuter
• данные указаны — plural

So the ending of указан tells you it matches a masculine singular noun.

Russian writes this as a topic-comment structure:

• Что касается удержанного налога, то он уже указан.

The first comma separates the introductory phrase Что касается удержанного налога from the main clause. The то belongs to the main clause and helps mark the transition.

This punctuation is standard for this construction.

So the pattern is:

• Что касается X, то Y.

It is not extremely formal, but it does sound more structured and somewhat more formal or written than very casual alternatives.

Common alternatives include:

• насчёт удержанного налога... = about the withheld tax...
• по поводу удержанного налога... = regarding the withheld tax...

But что касается is very common in business, official writing, explanations, and careful speech.

In this sentence, it fits well because the wording sounds administrative or financial.

Yes. A simpler version would be:

• Удержанный налог уже указан.

This means essentially the same thing: The withheld tax is already listed/indicated.

The version with Что касается ... , то ... does something extra: it highlights the tax as a topic, as if answering a concern like What about the withheld tax?

So:

• Удержанный налог уже указан. = plain statement
• Что касается удержанного налога, то он уже указан. = As for the withheld tax, it is already listed

Уже means already, and in Russian it commonly appears before the word or phrase it modifies.

So:

• он уже указан = it is already indicated/listed

This is the most natural placement here. Russian word order is flexible, but this order sounds neutral and standard.

Not exactly, even though they are closely related.

• удержанный налог = withheld tax
  This is a noun phrase using a full participle to describe the noun.

• налог удержан = the tax has been withheld
  This is a predicate using the short-form participle.

So in your sentence:

• удержанного налога describes the kind of tax being discussed
• указан tells what has happened to it in the document or statement

Usually, yes. In administrative, financial, and official contexts, указан often means something like:

• stated
• specified
• shown
• listed
• entered

So here the idea is probably that the withheld tax is already shown somewhere, such as in a form, report, statement, or payslip.

The exact best English translation depends on context, but grammatically указан is a very normal word for this kind of bureaucratic statement.