Shulchan Aruch O.Ch. 613:2-3
On Yom Kippur when washing Negel Vasser or after using the washroom, one should wash only the fingers and not the rest of the hand.
Many Poskim hold, that a sick person that is washing Netilas Yodayaim for bread, should wash his full hand.
The Mishna Verurah (sk7) writes Kohanim when washing their hands For Nesias Kapayim should wash their whole hand.
Why is it , Negel Vasser & after washroom, only fingers may be washed, & a Choleh or Kohen may wash their whole hand?
1) Tumas Yodayim,. Chazal were Goizer a Tumah on the hands only.
2) Ruach Ra Hashoreh Al Yodayim. Bad spirits that rests on the hands.
For Tumas Yodayim Chazal were Goizer to wash the whole hand.
To remove the Ruach Ra, all that is needed is wash away where
the Ruach is resting (fingers only)
Washing for bread is because of Tumas Yodayim (Serach Terumah)
Kohanim wash for same reason, not to have Yodayim Temeios.*
Washing Negel Vasser or after Washroom is to remove the Ruach Ra.
*Machlokes Rashi -Rambam