Add instance with signature field?

I am building a form where I have added a repeatable subform. I need a signature field within that subform. However, LiveCycle has prompted me with an error in the image below.

Several questions:

1. Why does it say a signature may become invalid in a dynamic pdf form? That's a conditional statement and does not explain what conditions have to be met to render the signature fields invalid.

2. Wouldn't moving the signature filed out of the subform prevent it from being replicated whenever the end user adds an instance?

3. What is a data signature? I am not familiar with this term.