@Ryan Bolger answer has a very good explanation. I wanted to include a more complete example for those who like to see what happens with various commands.
For example I use the following for the binddn distinguishedName: CN=auser,OU=IT Dev,OU=localdomain Users,DC=localdomain,DC=lan
-D 'CN=auser,OU=IT Dev,OU=localdomain Users,DC=localdomain,DC=lan'
or the UPN userPrincipalName: gitlab@nmm.lan
-D 'auser@localdomain.lan'
The following lines will produce the same output below
ldapsearch -x -h '192.168.0.10' -D 'CN=Auser,OU=IT Dev,OU=localdomain Users,DC=localdomain,DC=lan' -w password -b"cn=auser,OU=IT Dev,OU=localdomain Users,dc=localdomain,dc=lan" -s sub "objectclass=*"
or
ldapsearch -x -h '192.168.0.10' -D 'auser@localdomain.lan' -w password -b"cn=auser,OU=IT Dev,OU=localdomain Users,dc=localdomain,dc=lan" -s sub "objectclass=*"
The same output will be generated
# extended LDIF
#
# LDAPv3
# base <cn=auser,OU=IT Dev,OU=localdomain Users,dc=localdomain,dc=lan> with scope subtree
# filter: objectclass=*
# requesting: ALL
#
# auser, IT Dev, localdomain Users, localdomain.lan
dn: CN=GitLab,OU=IT Dev,OU=localdomain Users,DC=localdomain,DC=lan
objectClass: top
objectClass: person
objectClass: organizationalPerson
objectClass: user
cn: auser
givenName: auser
distinguishedName: CN=auser,OU=IT Dev,OU=localdomain Users,DC=localdomain,DC=lan
instanceType: 4
whenCreated: 20190221073536.0Z
whenChanged: 20190221080923.0Z
displayName: auser
uSNCreated: 108114404
memberOf: CN=groupofusers,OU=localdomain Groups,DC=localdomain,DC=lan
uSNChanged: 108116177
name: auser
userAccountControl: 66048
codePage: 0
countryCode: 0
primaryGroupID: 513
accountExpires: 9223372036854775807
sAMAccountName: auser
sAMAccountType: 805306368
userPrincipalName: auser@localdomain.lan
objectCategory: CN=Person,CN=Schema,CN=Configuration,DC=localdomain,DC=lan
dSCorePropagationData: 16010101000000.0Z
lastLogonTimestamp: 131952101637691018
# search result
search: 2
result: 0 Success
# numResponses: 2
# numEntries: 1