Страховая сумма указана в документе.

Страховая сумма is a set phrase in insurance language.

• страховая = insurance, insured, relating to insurance
• сумма = sum, amount

Together, страховая сумма means something like insured amount or insurance amount. In Russian, it is normal to use an adjective + noun combination where English might use a noun + noun structure.

Страховая is an adjective, and it must agree with сумма.

Since сумма is:

• feminine
• singular
• nominative here

the adjective also becomes:

• feminine
• singular
• nominative

So:

• masculine: страховой
• feminine: страховая
• neuter: страховое
• plural: страховые

Because сумма is the grammatical subject of the sentence.

The sentence is built like:

• Страховая сумма = the subject
• указана = is indicated / is specified
• в документе = in the document

So сумма stays in the nominative because it is the thing being described as specified in the document.

Указана is the short form passive participle, and in this sentence it works as the predicate.

It means something like:

• is indicated
• is specified
• has been stated

By contrast, указанная is the full form and usually behaves like an adjective before a noun:

• указанная сумма = the indicated amount / the amount mentioned

So:

• Страховая сумма указана в документе = The insured amount is specified in the document.
• указанная сумма = the amount that has been specified

Because it agrees with сумма, which is feminine singular.

Short-form passive participles change for gender and number:

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

Since сумма is feminine, Russian uses указана.

In Russian, the verb to be is usually omitted in the present tense.

So Russian says:

• Страховая сумма указана в документе

where English says:

• The insured amount is specified in the document

The idea of is is understood automatically.

Because в can take different cases depending on meaning.

Here it means in in the sense of location, so it takes the prepositional case:

• в документе = in the document

If there were motion into something, Russian would often use the accusative instead:

• положить в документ would sound odd with this noun, but with other nouns:
• положить в папку = put into the folder

In this sentence, there is no movement, only location, so в документе is correct.

Yes. Russian word order is more flexible than English, though the emphasis changes.

These are all possible:

• Страховая сумма указана в документе.
• В документе указана страховая сумма.
• Указана в документе страховая сумма. — possible, but less neutral

The most neutral version is the original.
If you put в документе first, you emphasize the location a bit more.

It is effectively passive in meaning.

The sentence focuses on the information being stated, not on who stated it.

• Страховая сумма указана в документе = The insured amount is specified in the document.

If you wanted a more active version, you would need to name the doer:

• В документе указали страховую сумму. = They specified the insured amount in the document.

Russian often uses passive-style constructions like указан / указана / указано / указаны in formal writing.

In this context, specified is usually the best choice.

The verb указать means:

• to indicate
• to point out
• to specify
• to state

So указана suggests that the amount is officially stated or listed in the document, not just casually mentioned.

In insurance or legal language, specified is often the most natural English equivalent.

A helpful stress guide is:

• страховАя сУмма указАна в докумЕнте

Approximate pronunciation:

• stra-ha-VA-ya SOOM-ma oo-ka-ZA-na v da-ku-MYEN-tye

The stressed syllables are:

• страховая
• сумма
• указана
• документе

This matters because Russian stress is unpredictable and can change from word to word.