code or plugin ( which one have a higher priority) in wordpress

my question is about a general scenario. i have written some code for user role and capabilities in wordpress and also install a plugin for that as well. when everr i change my code for capabilities to give more rights to the role that doesn't work but at the same time when i change the capabilities from the plugin then it works correctly.

So I want to know Is there any priority mechanism in WP which defines the priority for the plugin and the code that which one will be applied first.

thanks in advance