You can use both username field and source IP field filled in same rule.
Let's assume you have users John and Mary in username field and IPs 192.168.1.5 and 192.168.1.20 IPs in source IP field.
This means that traffic from users John or Mary will match to the rule if their IP is either 192.168.1.5 or 192.168.1.20
So if you look at the security policy page there is "OR" between items in same column and "AND" between columns.
Source user is either John OR Mary with source IP either 192.168.1.5 OR 192.168.1.20
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