Level 2 conclusion exam - custom codes for import question

Hi , I am currently struggling with one of the question of the conclusion exam (Level 2 Model Building). The question is on best practices with regards to creating custom codes for imports. The problem is that I don't remember anything about that in the videos/activities (took plenty of notes too but can't find the answer)... I am not asking for the direct answer but could anyone send me links to resources about that ?

Thanks in advance,

Mats