Monetized & Public means all visitors have access. You will receive payments from Coil Members. By default, non-members see a customizable message asking them to sign up. This option applies to entire pages.
Split Content is an option available within the Block/Gutenberg editor. You can set different visibility options to each block of a page. You can set a block so that all visitors can see the content (and still get paid when Coil Members visit), so that only Coil Members can see the content, or so that only non-Coil members can see the content. When you set a block to be visible to either Coil or non-Coil members, people will see a padlock next to the block when you publish your content. This padlock can be turned on or off by you, as the admin. Also, when a block is hidden from non-members, there’s a default message that will appear until the individual signs in to their Coil account. This message can be customized.
Check out these help pages in the Coil Help doc for more info, and let us know if you have any other questions!