I would put any transformation or string operation or creation of a new field in a preceding Load, rather than in a resident Load. I.e. if possible – always choose preceding Load.

But there are some transformations you cannot do in a preceding Load, e.g anything that you want to do after a Crosstable Load or after a Join. For these I would use a resident Load.

From

https://community.qlik.com/blogs/qlikviewdesignblog/2013/03/04/preceding-load