قواعد اللباس في الرسالة.

Not really. It is most naturally read as a noun phrase or title/headline, not a full sentence with a verb.

A full sentence in Arabic usually has either:

• a verb, or
• a complete subject + predicate structure.

Here, we just have:

• قواعد اللباس = dress code / clothing rules
• في الرسالة = in the message

So this works very well as a heading such as Dress code in the message.

This is because قواعد اللباس is an iḍāfa (إضافة), often called a construct phrase.

In an iḍāfa:

• the first noun usually does not take ال
• the second noun can make the whole phrase definite

So:

• قواعد = rules
• اللباس = the clothing / dress

Together:

• قواعد اللباس = rules of dress, or more naturally the dress code

Even though قواعد has no ال, the whole phrase can still be definite because اللباس is definite.

قواعد means rules.

Its singular is قاعدة = rule.

So:

• قاعدة = one rule
• قواعد = rules

This is a broken plural, which is very common in Arabic. That means the plural is not formed just by adding a regular ending; the internal shape of the word changes.

اللباس literally means clothing, attire, or dress.

In this phrase, قواعد اللباس is a natural way to say:

• dress code
• clothing rules
• rules of dress

So the meaning is not about one specific item of clothing, but about how people are supposed to dress.

في means in.

الرسالة means the message, the letter, or sometimes the communication, depending on context.

So:

• في الرسالة = in the message

Because في is a preposition, the noun after it is in the genitive case.

In fully vowelled Arabic, you would write:

• في الرِّسالةِ

So the final -i sound on الرسالةِ comes from the preposition في.

Because ر is a sun letter.

When ال comes before a sun letter, the l sound of ال assimilates to the next consonant.

So:

• written: الرسالة
• pronounced: ar-risālah

In fully vowelled writing, this is shown with a shadda:

• الرِّسالة

The same kind of thing happens with other sun letters such as ت، د، س، ش، ن and others.

A careful full-vowel version would be:

قَواعِدُ اللِّباسِ في الرِّسالَةِ

Word by word:

• قَواعِدُ = qawāʿidu
• اللِّباسِ = al-libāsi
• في = fī
• الرِّسالَةِ = ar-risālati

A few notes:

• قواعدُ has ḍamma here if it stands as the main noun of the phrase.
• اللباسِ is genitive because it is the second part of the iḍāfa.
• الرسالةِ is genitive because it comes after في.

In normal everyday writing, these short vowels are usually omitted.

Because it is the second noun in an iḍāfa construction.

In Arabic iḍāfa:

• the first noun is the thing possessed or described
• the second noun is the possessor or specifying noun
• the second noun is genitive

So:

• قواعدُ = rules
• اللباسِ = of dress / of clothing

That is why you get:

• قواعدُ اللباسِ

This literally means rules of clothing/dress.

Yes. A very literal translation might sound awkward in English, so more natural translations include:

• Dress code in the message
• The dress code in the message
• Clothing rules in the message

The best choice depends on context. If this appears as a heading, Dress code in the message or simply Dress code may sound most natural.

Normally, it is understood to describe the whole phrase:

• قواعد اللباس = dress code
• في الرسالة = in the message

So the overall sense is:

• the dress code that appears in the message
• the clothing rules mentioned in the message

In other words, في الرسالة is most naturally attached to the whole noun phrase, not just to اللباس by itself.