The difference of two bitstreams in PYNQ build environment

I am trying to build PYNQ on my snickerdoodle board with 2019.1 on ubuntu 18.04. I realized that I have to use prebuilt rootfs, but still have some trouble on the built process. I found that vivado was called to built two bitstream when doing test on Pynq-Z2:

petalinux_bsp/hardware_project/build_bitstream.tcl was called in
make boot_files BOARDS=Pynq-Z2

base/build_bitstream.tcl was called in
make images BOARDS=Pynq-Z2

what is the difference of theses two bitstreams?

Thank you very much.

the petalinux_bsp hw project is used, as the path suggests, to create the petalinux bsp used to build the image.

The second tcl is the actual base overlay shipped on the Z2 board that provides the functionalities to run the included notebooks.

Hope this clarifies your doubts.

EDIT: the tcl files you are referring to in the post are just wrapper tcl though, that contains the code to build the respective bitstreams. The actual tcl files with the two designs are pynqz2.tcl in petalinux_bsp/hardware_project/ and base.tcl in base/