En el anexo aparecen los datos que faltaban en la solicitud.

Spanish allows much more flexible word order than English. In this sentence, aparecen los datos is a very natural formal way to say that the data appear / are shown / are listed in the attachment.

The normal English-style order would also be possible:

Los datos que faltaban en la solicitud aparecen en el anexo.

But En el anexo aparecen... sounds especially natural in administrative or written Spanish because it puts the location first: In the attachment, the missing information appears.

Because the subject is los datos, which is plural.

• el dato = the piece of information / datum
• los datos = the data / the information

So the verb must agree with los datos:

• aparece = singular
• aparecen = plural

Even though anexo comes earlier in the sentence, it is not the subject. The subject is los datos.

In this context, anexo usually means attachment, appendix, or enclosure, depending on the kind of document.

So En el anexo could be understood as:

• In the attachment
• In the appendix
• In the enclosed document

In administrative Spanish, anexo is very common in forms, applications, and official paperwork.

Because anexo is a masculine noun: el anexo.

So:

• el anexo
• en el anexo

The ending -o often suggests masculine nouns, and here that pattern holds true.

Yes, but in everyday Spanish, los datos often means the information or the details, not just data in a technical sense.

In administrative contexts, datos can refer to things like:

• name
• address
• ID number
• contact details
• other required information on a form

So here los datos que faltaban means something like the missing details/information.

Que faltaban means that were missing.

It is a relative clause describing los datos:

• los datos = the data / details
• que faltaban = that were missing

So the structure is:

los datos que faltaban en la solicitud
= the information that was missing from the application/request

The imperfect faltaban is used because it describes a past situation or state: those details were missing at the time of the application/request.

It is not presenting a completed action like faltaron, but rather a condition that existed:

• faltaban = were missing
• faltaron = went missing / were absent in a more completed-event sense
• faltan = are missing now

So faltaban fits the idea that the request had missing information, and that information now appears in the attachment.

Solicitud usually means application, request, or form, depending on context.

In official or bureaucratic Spanish, solicitud is very common for:

• an application form
• a formal request
• a submitted document asking for something

So en la solicitud likely means in the application or on the application form.

Because the sentence is referring to information that was missing in/from the application itself.

• en la solicitud = in the application / on the application
• de la solicitud = of the application

Here the meaning is about where the missing information was absent, so en is the natural preposition.

In English we might say either missing from the application or missing on the application form, but Spanish commonly uses en in this kind of sentence.

Literally, aparecen means appear, but in this formal document context it often means:

• are shown
• are listed
• are included
• can be found

So while the literal translation is fine, the practical meaning is closer to:

The missing details are included in the attachment.

That is often how a native English speaker would naturally express it.

Yes. The best translation depends on context.

If this is an email or submitted document with extra pages, attachment is often the best choice.
If it is part of a longer report or formal document, appendix may be better.
If it is an official paper sent together with the main document, enclosure can also work.

So anexo is a flexible administrative word, and you choose the English equivalent from context.

Yes, that would be grammatically correct, and it sounds natural too.

• En el anexo aparecen los datos... = The details appear / are listed in the attachment.
• En el anexo están los datos... = The details are in the attachment.

The version with aparecen sounds a bit more formal or document-like. The version with están is simpler and more direct.

Spanish uses definite articles more often than English does.

Here they are natural because the sentence refers to specific things already known in the situation:

• el anexo = the attachment already mentioned or attached
• los datos = the specific missing details
• la solicitud = the specific application/request being discussed

In English, we might sometimes omit an article or use a different structure, but in Spanish these articles sound completely normal.