What are passive images? - creating templates with new ANTS (nipype) sMRI

I am currently looking into nypipe’s sMRI ants workflow to create a template from multiple T1w images, using “ANTSTemplateBuildSingleIterationWF” (see here: nipype docs).

Unfortunately, the documentation or examples don’t explain much, besides specifying in- and output fields. As input, three fields are required:
inputspec.images :
inputspec.fixed_image :

I don’t understand what is meant by “passive images”, and what images are supposed to be fed in, besides those that serve as a basis for the template. In the example given, the file names of the passive images have an additional “inv” tag (possibly inverse?), but why would we want to feed in additional, inverted images?

I appreciate your help, thanks so much in advance!