When a signer hovers over a tag, the tooltip appears.  

You use "Recipients can collaborate" when you want signers to share fields.  Normally, when you add a text box for a signer, only that signer can edit the value in the text box.  When collaboration is enabled, multiple signers can edit the value in the text box.  You might use this option when signers are trying to agree on a payment amount, for example.  Signer 1 might enter 20 for the amount, signer 2 might change it to 30, the envelope could go back to signer 1 to either initial the change or accept the current value.

The required field option applies to signers.  The "mandatory" option applies to senders.  If, for example, you have a template with a text box marked as mandatory, the sender of that template will not be able to delete the field from the document because it is mandatory.  

This always ends up as the first result when I type "docusign sender permissions" into Google so I'll just add (pasted from here: https://support.docusign.com/en/guides/ndse-user-guide-locked-templates):

Restrict changes

• This setting guarantees the field will be present on all envelopes created with the template, exactly as it was set up on the template. For all field types, the sender cannot delete the field or change the recipient, or modify the placement, size, data label, conditional logic, or any other field property setting.
• For text, checkbox, and radio button fields, the sender can enter text and make selections. For note fields, the sender can enter the note text. However, since the note field size is fixed, only the text that fits into the field is included and visible to the recipient.

Mandatory

• This setting guarantees the field will be present on all envelopes created with the template. The sender cannot delete a mandatory field. They can, however, make other changes to the field, such as moving, resizing, or setting any other field properties.

Stumbled upon this when googling for help.

After signer 2 revises a collaborative field, is signer 1 supposed to get an email? When testing, I ran into an issue where signer 1 never got any email after changes were made.

Thanks!