betreffend
relevant
As an adjective or participle with the meaning 'the respective' or 'the one in question'.
Bitte leiten Sie die E-Mail an den betreffenden Mitarbeiter weiter.
Please forward the email to the relevant employee.

concerning / relevant
Loading your saved state.
relevant
As an adjective or participle with the meaning 'the respective' or 'the one in question'.
Bitte leiten Sie die E-Mail an den betreffenden Mitarbeiter weiter.
Please forward the email to the relevant employee.

'Betreffend' is the present participle of the verb 'betreffen' (to concern, to affect). It therefore literally means 'concerning' or 'referring to'.
Alle Dokumente, die diesen Fall betreffen, müssen gesammelt werden.
All documents that concern this case must be collected.
The function of 'betreffend' depends on its position. If it comes before a noun, it is an adjective and is declined. If it comes after a noun, it is a preposition.
Der betreffende Sachbearbeiter ist heute nicht im Haus.
The relevant case worker is not in the office today.
The use as a postposed preposition ('Ihren Antrag betreffend' — concerning your application) is typical of formal written language, especially in administration or business.
Die neuen Regelungen betreffend, gibt es noch einige Unklarheiten.
Concerning the new regulations, there are still some uncertainties.
When the phrase with 'betreffend' stands at the beginning of the sentence, the conjugated verb follows directly afterward (inversion). The noun before 'betreffend' is in the accusative.
Den von Ihnen genannten Vorfall betreffend, haben wir eine Untersuchung eingeleitet.
Concerning the incident you mentioned, we have initiated an investigation.
over / about
'über' and 'betreffend' can both indicate a topic. 'über' is very normal in everyday language, especially with conversations, texts, thoughts, or news. 'betreffend' means more like 'as far as ... is concerned' and sounds more formal. It often appears in letters, emails, or official information and can stand after the noun, for example 'die Wohnung betreffend' (concerning the apartment). In normal conversations, 'über' is usually more natural. 'betreffend' fits better when something should sound factual or written.

Betreffend die Reparatur der Heizung habe ich heute eine E-Mail bekommen.
Concerning the repair of the heating system, I received an email today.
Wir haben lange über die neuen Nachbarn gesprochen.
We talked for a long time about the new neighbors.