Does one need to cover the shel yad tefillin with his sleeve from jacket/tallit as well as the protective box?
I see lots of people who do cover and people who do not and therefore wondering what the actual halachic requirement is.
Essentially the tefillin shel yad does not have to be covered. The main reason we cover it with the inner box is in order to protect the corners from getting nicked and scrapped. I once heard that there is a prefferance to cover one’s sleeve, and make oneself as presentable as possible, because when standing shmona esrei we are standing in from of the king. However this is not a halacha but a prefferance, and both ways are acceptable. There are also other reasons, see Sharei Teshuva 27-11, and Piskei Tesuvos 27-23.