Что касается недоимки, то она указана в личном кабинете налогоплательщика.

Что касается is a fixed expression meaning as for, regarding, or when it comes to.

So:

• Что касается недоимки... = As for the arrears/underpayment...

It is very common in formal and semi-formal Russian, especially when introducing a topic before saying something about it.

A useful pattern is:

• Что касается + genitive, то ...
• As for + noun, ...

Because касаться normally takes the genitive case.

The dictionary form is:

• недоимка = nominative singular

But after что касается, you need the genitive:

• что касается недоимки

So this is not plural here. It is genitive singular.

Недоимка is a fairly formal legal/financial word. It usually means an unpaid amount owed, especially a tax arrears or tax underpayment.

In everyday English, the exact translation depends on context:

• tax arrears
• outstanding tax debt
• underpaid tax amount

In this sentence, because of налогоплательщика, it clearly belongs to the tax/official context.

The то in Что касается недоимки, то... is a particle that helps mark the main statement after the topic has been introduced.

So the structure is:

• Что касается X, то Y
• As for X, Y

It does not always need a direct English translation. It mainly helps organize the sentence.

Without то, the sentence would still be understandable:

• Что касается недоимки, она указана...

But то makes the topic-comment structure clearer and sounds especially natural in formal writing.

Because недоимка is a feminine singular noun, and the pronoun referring back to it must agree with it:

• недоимка → она

In English, we would normally say it, but Russian pronouns follow grammatical gender, not natural gender here.

So:

• недоимка = feminine
• therefore она указана

Указана is the short-form passive participle of указать.

Here it means something like:

• is indicated
• is listed
• is specified

It agrees with она, so it is in the feminine singular form:

• masculine: указан
• feminine: указана
• neuter: указано
• plural: указаны

This form is very common in formal written Russian.

So:

• она указана = it is indicated/listed

Because указана must agree with the noun it refers to.

The thing being referred to is:

• недоимка — feminine singular

So the correct form is:

• она указана

If the subject were neuter, then указано would be possible. For example:

• сообщение указано would be wrong, because сообщение is neuter, so it would have to be указано only if that were the correct passive construction for that context.

In this sentence, feminine agreement is required.

Because в here means in, and it refers to a location, not motion toward somewhere.

When в means location, Russian normally uses the prepositional case:

• в кабинете = in the account/in the office

That is why you get:

• личный кабинет → в личном кабинете

The adjective also changes:

• личный → личном

So the phrase means in the personal account.

No. In modern Russian official and online usage, личный кабинет usually means a person’s online account or personal portal area on a website.

So in this sentence it means something like:

• the taxpayer’s online account
• the taxpayer’s personal account/dashboard

It does not mean a literal room or office here.

Налогоплательщика is the genitive singular form of налогоплательщик meaning taxpayer.

Here it shows possession or association:

• личный кабинет налогоплательщика = the taxpayer’s personal account

Russian often expresses this idea with noun + genitive noun instead of using an apostrophe like English does.

So literally it is something like:

• personal account of the taxpayer

Russian often uses word order to organize information as topic → comment.

Here:

• Что касается недоимки introduces the topic
• то она указана... gives the information about that topic

This is more like:

• As for the tax arrears, it is listed...

If you started with Недоимка указана..., that would also be possible, but it would sound more direct and less like a topic shift.

The version in your sentence is especially typical of formal explanatory language.

Because Что касается недоимки is an introductory topic-setting part, and the main clause begins after it:

• Что касается недоимки, то она указана...

The comma separates:

1. the topic phrase
2. the main statement

This punctuation is standard with this kind of structure.

Yes, very much so.

Several things make it sound official or administrative:

• что касается = formal topic-introducing phrase
• недоимка = legal/financial term
• указана = formal written style
• личный кабинет налогоплательщика = bureaucratic/administrative vocabulary

A more casual everyday sentence would usually be phrased differently. This one sounds like something from a government website, tax notice, or official explanation.