From b2evolution manual

Apply rendering defines when a Renderer Plugin should apply to rendering.

This is currently only used when Items get displayed ("rendered").

This setting can be changed in the Plugin Settings admin and defines how the plugin gets presented in the "Renderers" fieldset when creating or editing an existing item.

[edit] Possible values

• stealth: gets always used, but not displayed as option
• always: gets always used, and displayed as disabled checkbox
• opt-out: enabled by default, can get unchecked
• opt-in: disabled by default, can get checked
• lazy: checkbox gets displayed, but is disabled. This usually means that the plugin decides it itself (by looking at the content)
• never: cannot get used as a Renderer Plugin