يشمل أمر الدفع المستديم القسط.

Because Arabic often uses verb–subject–object order in formal writing.

So the structure here is:

• يشمل = the verb
• أمر الدفع المستديم = the subject
• القسط = the object

This is very normal in Modern Standard Arabic.
You could also say أمر الدفع المستديم يشمل القسط. That is also correct.

يشمل is the imperfect form of the verb شَمِلَ / يَشْمَلُ.

Here it is:

• 3rd person
• masculine
• singular

So it means something like it includes or it covers.

Arabic imperfect verbs can refer to the present, habitual action, or sometimes near future depending on context.

Because its subject, أمر, is:

• singular
• masculine

So the verb agrees with it.

In this sentence, the core subject is أمر inside the phrase أمر الدفع المستديم.

Because this is an iḍāfa construction, often called a construct phrase.

أمر الدفع literally means order of payment.

In an iḍāfa:

• the first noun is the thing possessed or defined: أمر
• the second noun completes it: الدفع

Important rule:

• the first noun in an iḍāfa normally does not take ال
• the whole phrase can still be definite because of the second noun

So أمر الدفع can be definite even though أمر itself does not have ال.

Only the first noun in the iḍāfa is blocked from taking ال.
The second noun can be definite or indefinite as needed.

So:

• أمر دفع = an order of payment / a payment order
• أمر الدفع = the order of payment / the payment order

That is why الدفع can have ال here.

Because Arabic adjectives normally come after the noun or noun phrase they describe.

Here, المستديم is an adjective placed after the whole phrase أمر الدفع.

That is the normal Arabic pattern:

• noun first
• adjective second

So Arabic says something structurally like:

• payment order standing

rather than putting the adjective before the noun as English usually does.

Because adjectives in Arabic agree with the noun they describe in definiteness.

The phrase أمر الدفع is definite, so the adjective must also be definite:

• أمر الدفع = definite
• المستديم = therefore also definite

This is part of normal adjective agreement in Arabic.
Adjectives also agree in:

• gender
• number
• case
• definiteness

This is a very good question, because in unvowelled Arabic writing it can sometimes look ambiguous.

In context, the intended meaning is usually that المستديم describes the whole idea of أمر الدفع as a banking term, so:

• أمر الدفع المستديم = the standing payment order

If the text were fully vowelled, the case ending would help show the grammar more clearly.

A learner should know that Arabic often leaves this kind of detail to context when short vowels are not written.

Because يشمل is a transitive verb, so it takes a direct object.

Here:

• أمر الدفع المستديم = the thing doing the action
• القسط = the thing being included

If full case endings were written, you would see that more clearly:

• أمرُ = nominative, so it is the subject
• القسطَ = accusative, so it is the direct object

In normal unvowelled writing, word order and meaning usually make this clear.

Because يشمل takes a direct object in Arabic.

So Arabic says:

• يشمل القسط

not something like:

• يشمل بـالقسط
• يشمل إلى القسط

This is similar to English includes the installment, where no preposition is needed.

Yes. Arabic allows more than one word order.

You can say:

• يشمل أمر الدفع المستديم القسط
• أمر الدفع المستديم يشمل القسط

Both are grammatical.

The first version, with the verb first, is very natural in formal MSA.
The second version puts more focus on the subject.

A fully vowelled version could be written as:

يَشْمَلُ أَمْرُ الدَّفْعِ المُسْتَدِيمُ القِسْطَ

This shows the grammatical roles more clearly:

• يَشْمَلُ = verb
• أَمْرُ = subject, nominative
• الدَّفْعِ = genitive because it is the second part of the iḍāfa
• المُسْتَدِيمُ = adjective agreeing with أمر
• القِسْطَ = direct object, accusative

Also, in pronunciation, الدفع is said ad-dafʿ, because د is a sun letter, so the l sound of ال assimilates.