What does the less sign (<) mean in dovecot.conf in front of ssl certificate path?

Question

A couple of years ago I've configured a mail server with exim4 + dovecot 2.2.10 on debian and recently have been tweaking dovecot config. I use a self-signed x509 openSSL certificate, the paths to the certificate and the key are given as

ssl_cert = </etc/ssl/certs/dovecot.pem"

Notice the < in front of the full path.

I tried removing it to see the effect, and here's what I got in dovecot.log:

dovecot: imap-login: Fatal: Couldn't parse private ssl_key: error:0906D06C:PEM routines:PEM_read_bio:no start line: Expecting: ANY PRIVATE KEY

Putting the < back got rid of it, but I can't find the meaning behind this sign. Some manuals give these paths without the <, though the official dovecot documentation uses it.

Answer 1

I would expect it needs the < to tell it to set the config value to the contents of the file instead of to the actual string /etc/ssl/certs/dovecot.pem (which obviously isn't a valid SSL certificate).