In this SE you will prove your writing skills in formal letters. Your language competence needs to be on a B1+ level, which means that...
... you can convey information and ideas on abstract as well as concrete topics, check information, and ask about or explain problems with reasonable precision.
... you can write text that is clear and well-organised.
... you can describe your feelings, experiences and opinions on things you have done or seen.
... you can describe the advantages and disadvantages of issues you have an interest in.
... you can summarise facts and events.
... you have enough vocabulary to convey the main idea about various topics with precision.
... your grammar is controlled and generally good. Errors may occur in advanced structures, but do not hinder meaning.
... your spelling, punctuation and layout are accurate enough to be followed.