Что добавляет тег -i к команде, такой как:
ssh -i path-to-pem-file ubuntu@ip-address
На странице man
-i identity_file Selects a file from which the identity (private key) for public
key authentication is read. The default is ~/.ssh/identity for
protocol version 1, and ~/.ssh/id_dsa, ~/.ssh/id_ecdsa,
~/.ssh/id_ed25519 and ~/.ssh/id_rsa for protocol version 2.
Identity files may also be specified on a per-host basis in the
configuration file. It is possible to have multiple -i options
(and multiple identities specified in configuration files). If
no certificates have been explicitly specified by the
CertificateFile directive, ssh will also try to load certificate
information from the filename obtained by appending -cert.pub to
identity filenames.