[HarfBuzz] Question about zero width glyphs in shaping output

[Deepak Jois]

Maybe this is a bit related to Khaled’s question earlier about control
characters inside ligatures, but I wanted to start a new thread.

When I shape text with Noto Nastaliq, I notice a bunch of zero-width
glyphs generated

$> hb-unicode-encode U+06CC,U+06C1 |  hb-shape notonastaliq.ttf
[HehFin=1+472|TwoDotsBelowNS=0 at 310,-383+0|sp2=0+0|BehxIni.outS1=0 at 0,-68+731]

1. What is the purpose of these zero-width glyphs?

2. If I am rendering the shaped output to a PDF file (for e.g. when
using Harfbuzz with LuaTeX), do I need to care about these zero-width
glyphs at all? How will they affect rendering