Composer 4.3 sp2
Initial Visibility can be set to:
What's the practical difference between the last two options?
I haven't actually tested this to be sure, but usually:
For example, if you have a block, if it's invisible, you will get a blank area on the form. But if it's hidden, the block will be collapsed and the rest of the form will move up. The same applies with fields that are displayed side by side, for example.
The actual decision of hidden vs invisible can be tricky.
In most cases, and especially for blocks, you will want to use hidden, otherwise you will end up with large amounts of white space in a form. However, sometimes you will want the surrounding fields to maintain their relative positions even though the field is not shown, and in that case you would use invisible.
Note that when you use a visibility rule, it actually uses hidden rather than invisible, since usually you want the form to collapse when something is not shown. I think this can be tweaked using an advanced property.