Prüfe den Termin, sei es im Onlinekatalog oder auf der Quittung.

Prüfe is the imperative form, so it means Check!

More specifically, it is the command form used for du (informal singular, when speaking to one person).

From prüfen, the du imperative can be:

• Prüf!
• Prüfe!

Both are correct. Prüfe sounds a bit more careful, formal, or written.

Compare:

• Prüfe den Termin. = informal command to one person
• Prüft den Termin. = informal command to several people
• Prüfen Sie den Termin. = formal command

Because prüfen takes a direct object, and direct objects are usually in the accusative case.

The noun Termin is masculine:

• nominative: der Termin
• accusative: den Termin

So:

• Der Termin ist wichtig. = the appointment/date is important
• Prüfe den Termin. = check the appointment/date

Sei es ... oder ... is a fixed expression meaning something like:

• whether ... or ...
• be it ... or ...

In this sentence, it introduces two possible places where you can check the information:

• im Onlinekatalog
• auf der Quittung

So the structure means: do this, no matter which of these two places you use.

It is a fairly formal or written expression.

Here, sei is not the normal present-tense form of sein. It is the Konjunktiv I form, used in the fixed expression sei es.

So this is not the same as saying it is. It is an idiomatic structure meaning be it or whether it be.

That is why ist es would not sound right here.

You can think of sei es as a chunk to memorize:

• sei es A oder B = whether A or B
• sei es aus Zeitgründen = be it for reasons of time

In this expression, es is basically a dummy pronoun. It does not directly refer to Termin.

So even though Termin is masculine, the phrase still stays sei es.

That is because sei es works as a fixed idiom. You do not normally change it to match the gender of another noun in the sentence.

Because German uses different prepositions depending on the kind of place or medium being talked about.

• im Onlinekatalog = in the online catalog
• auf der Quittung = on the receipt

This is mostly idiomatic usage:

• information is found in a catalog
• information is found on a receipt

Also:

• im = in dem
• der Quittung is dative, because auf here describes a location, not movement

Compare:

• im Buch = in the book
• auf dem Zettel = on the note
• auf der Website = on the website

The comma separates the main command from the added expression that gives alternatives.

Main part:

• Prüfe den Termin

Added part:

• sei es im Onlinekatalog oder auf der Quittung

German often uses commas before this kind of inserted or explanatory structure. Here, the second part is not a new independent sentence; it is an attached phrase explaining where the checking can be done.

Yes, it sounds somewhat formal or written.

Two things contribute to that:

• Prüfe instead of the shorter Prüf
• the expression sei es ... oder ...

In everyday spoken German, someone might say something simpler, such as:

• Prüf den Termin im Onlinekatalog oder auf der Quittung.

That version is more direct and conversational.

Yes. You could simply say:

• Prüfe den Termin im Onlinekatalog oder auf der Quittung.

That is grammatical and more straightforward.

Adding sei es makes the sentence sound more formal and slightly more emphatic, with the sense of:

• whether in the online catalog or on the receipt
• be it in the online catalog or on the receipt

So sei es is not required for basic meaning, but it changes the style.

Because German very often forms compound nouns by writing them as one word.

Here:

• online
  • Katalog → Onlinekatalog

This is very normal in German. The last part usually determines the grammatical gender, so because Katalog is masculine, Onlinekatalog is also masculine:

• der Katalog
• der Onlinekatalog

You will see this pattern everywhere in German, for example:

• Buchhandlung
• Zahnarzttermin
• Kundenservice

It is understandable and correct, but it is more common in written German, instructions, formal speech, or polished style than in casual everyday conversation.

In everyday speech, many people would prefer simpler wording, for example:

• Prüf den Termin im Onlinekatalog oder auf der Quittung.
• Du kannst den Termin im Onlinekatalog oder auf der Quittung prüfen.

So it is useful to recognize sei es ... oder ..., but you do not need to use it often in casual conversation.