Что касается налогового периода, то он указан в налоговой декларации.

Что касается is a set phrase meaning as for, regarding, or with regard to.

So:

Что касается налогового периода...
= As for the tax period...
= Regarding the tax period...

It is very common in formal and semi-formal Russian, especially in explanations, official language, and writing.

Because что касается requires the genitive case.

The base form is:

• налоговый период = tax period

But after что касается, both words change to genitive singular:

• налогового периода

So this is not random; it is a grammar pattern:

• что касается + genitive

Examples:

• Что касается цены... = As for the price...
• Что касается документа... = As for the document...

In this sentence, то helps mark the second part of the structure:

• Что касается X, то Y.

This is a very common Russian pattern. It separates the topic from the comment.

So:

• Что касается налогового периода, то он указан...

literally feels like:

• As for the tax period, then it is indicated...

In natural English, we usually do not translate то directly. It mainly gives the sentence a clearer, more formal structure in Russian.

The sentence would still be understandable without it:

• Что касается налогового периода, он указан в налоговой декларации.

But with то, it sounds more standard and polished, especially in formal style.

Он refers to налоговый период.

Since период is a masculine singular noun, the pronoun is also masculine singular:

• период → он

So:

• то он указан = it is indicated

Russian pronouns agree with the grammatical gender of the noun, not with natural gender or with English habits.

Указан is the short-form passive participle of указать and means indicated, specified, or stated.

Here it agrees with налоговый период, which is masculine singular:

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

Examples:

• Период указан. = The period is indicated.
• Дата указана. = The date is indicated.
• Имя указано. = The name is indicated.
• Данные указаны. = The data are indicated.

Also, the correct full form would be указанный, not указаный. But that would be used differently:

• указанный период = the indicated/specified period

In your sentence, Russian uses the short form because it is part of the predicate:

• он указан = it is indicated

It is in the prepositional case because it follows в in the meaning in (location).

Base form:

• налоговая декларация = tax declaration / tax return

After в meaning in, it becomes:

• в налоговой декларации

So the sentence means:

• in the tax declaration
• or more naturally in English, in the tax return

This is a normal location pattern:

• в документе = in the document
• в отчёте = in the report
• в декларации = in the declaration

Often yes, in many contexts налоговая декларация is best translated as tax return.

Word-for-word, it is:

• налоговая = tax
• декларация = declaration

But in real English usage, tax return is often the most natural equivalent.

So depending on context, this phrase could be understood as:

• tax declaration
• tax return

Russian often uses topic-first structures to organize information. Here the speaker first introduces the topic:

• Что касается налогового периода... = As for the tax period...

Then says something about it:

• ...то он указан в налоговой декларации. = ...it is indicated in the tax declaration.

This structure is especially common in formal explanations, instructions, and bureaucratic language. It sounds a bit more organized and deliberate than simply saying:

• Налоговый период указан в налоговой декларации.

Both are correct, but the version with Что касается... highlights the topic more strongly.

Yes, it sounds fairly formal and somewhat official or bureaucratic.

Reasons:

• Что касается... is common in formal exposition.
• налоговый период
• налоговая декларация
• указан in this passive-style construction is typical of official language.

A more direct everyday-style sentence might be:

• Налоговый период указан в декларации.

But your original version is perfectly normal in official or administrative contexts.

Yes, Russian word order is flexible, but not all versions sound equally natural.

The original:

• Что касается налогового периода, то он указан в налоговой декларации.

This is very natural and clear.

You could also say:

• Налоговый период указан в налоговой декларации.
• В налоговой декларации указан налоговый период.

These are also correct, but the emphasis changes slightly.

The original version is best if the speaker wants to say as for the tax period... and then comment on it.

Yes. Even though указан comes from a participial form, in sentences like this it usually has a present-state meaning in English:

• он указан = it is indicated / it is stated

So the idea is not really it was indicated as a past event, but rather it appears there / it is listed there now.

This is very common in Russian official style:

• адрес указан в договоре = the address is stated in the contract
• срок указан в заявлении = the срок is specified in the application

So in practice, you can usually understand он указан as it is specified/stated.