If partial blocks is enabled, the Special:Block form acts as if the "Editing" checkbox is checked, even when it is unchecked.

Examples:

• The "sitewide" radio is selected, but the "Editing" checkbox is unchecked. This should create a partial block that doesn't affect editing, but actually creates a sitewide block.
• The "Partial" radio is selected, and page "Foo" is specified in the "Pages" input, but the "Editing" checkbox is unchecked. This should create a partial block that doesn't affect editing, but actually creates a partial block that prevents the target user from editing page "Foo".

Additionally, the "Editing" checkbox is always checked on Special:Block/Username