Question about incremental backups

This question is about making and storing images on network drives.

When making images, I am prompted to either choose a fileaname to create a full, or choose an existing file to make an incremental.
However, the program allows me to choose a new filename, but then choose to append.

Lets say I do the following:

- choose to create an image of the local computer with a new file name on a networked computer where no other .tib files exist

-choose to append at the next screen in the wizard

The program then begins the image creation process. How can it make an incremental without a full image file to reference?

Likewise, if I choose to create an image on a computer in a folder where other .tib files DO exist, does the program use one of them to make an incremental?

Thanks for any help.