Need clarity - logstash 'filter' patterns

Elastic Stack Logstash
1

In the docs and various examples, I'm seeing if statements in Input section, like:

if [message] =~ /dovecot: (imap|pop3|managesieve)-login

Apart from [message], what other fields can if take? I don't have much understanding about this. Is any documentation available?

Thanks.

2

https://www.elastic.co/guide/en/logstash/current/event-dependent-configuration.html#conditionals

3

Basically you can put a reference to a field, or a value.