Makom Petur, in general, are those places and objects that are not found in Reshus ha'Yachid, and do not fulfill the requirements that would make them Reshus ha'Yachid, Reshus ha'Rabim, or Karmelis in their own right. That is, they are not 4 x 4 Tefachim wide and are not walked upon by everyone like the rest of Reshus ha'Rabim. Transferring from them to one of the other three Reshuyos is permitted (as long as one does not transfer from Reshus ha'Yachid to a Makom Petur, and then from there to a Reshus ha'Rabim, or v.v.). Some examples of Makom Petur are a doorstep with an area less than four by four Tefachim, a pole (or fire-hydrant) in Reshus ha'Rabim with a surface area on top of less than four by four Tefachim. (If the pole is exactly 9 Tefachim tall, however, it is used frequently by those who walk through Reshus ha'Rabim, and considered to be a part of Reshus ha'Rabim.)

The airspace above a height of ten Tefachim inside Reshus ha'Rabim is also considered to be a Makom Petur, if something were to stop in mid-air there. The Gemara gives some examples of the practical applications of this latter Makom Petur

(Actually, by Torah law, there is no such thing as a Karmelis. Thus, every Karmelis is also defined by the Torah as a Makom Petur.)

See Shabbos, Charts #1 and #2, for more details (attached).

M. KORNFELD