On the majority of the user picker fields, we can enter either part of the email address or part of the Display Name, and a match of either one will be returned.

Also, for the user picker used on the Space Permissions, we do not have consistent results when searching for the user.
Depending on how is typed the values, it might not return any results.

Considering the sample user with display name "jackie williams" and email jwilliams@mycompany.com:

I will represent space with a (.)

search	result
copy&paste jwilliams@mycompany.com
j
ja
jac
jack
jacki
jackie
jackie(.)
jackie(.)w
jackie(.)wi
jackie(.)wil
jackie(.)will
jackie(.)willi
jackie(.)willia
jackie(.)william
jackie(.)williams
jackie(.)williams(.)
J
Ja
Jac
Jack
Jacki
Jackie
Jackie(.)
Jackie(.)W
Jackie(.)Wi
Jackie(.)Wil
Jackie(.)Will
Jackie(.)Willi
Jackie(.)Willia
Jackie(.)William
Jackie(.)Williams
Jackie(.)Williams(.)
j
jw
jwi
jwir
jwill
jwilli
jwillia
jwilliam
jwilliams
jwilliams(.)
jwilliams@
jwilliams@m
jwilliams@my
jwilliams@myc
jwilliams@myco
jwilliams@mycom
jwilliams@mycomp
jwilliams@mycompa
jwilliams@mycompan
jwilliams@mycompany
jwilliams@mycompany.
jwilliams@mycompany.c
jwilliams@mycompany.co
jwilliams@mycompany.com
jwilliams@mycompany.com(.)
J
Jw
Jwi
Jwil
Jwill
Jwilli
Jwillia
Jwilliam
Jwilliams
Jwilliams(.)
Jwilliams@
Jwilliams@m
Jwilliams@my
Jwilliams@myc
Jwilliams@myco
Jwilliams@mycom
Jwilliams@mycomp
Jwilliams@mycompa
Jwilliams@mycompan
Jwilliams@mycompany
Jwilliams@mycompany.
Jwilliams@mycompany.c
Jwilliams@mycompany.co
Jwilliams@mycompany.com
Jwilliams@mycompany.com(.)