MB4 199b: Makom Patur on Shabbos (345:19); Carrying on Shabbos (346:1)

A makom patur has to be at least 4 tefachim in both length and width.

Midoraita, one is only chayav for transferring between reshut hayachid and reshut harabim.

Unlike a karmelit, one can transfer between between a makom ptur and a reshut hayachid and between a makom petur and a reshut harabim.

Transferring, via a makom petur,  between a reshut harabim and a reshut hayachid is פטור אבל אסור.

Standing in a makom petur and taking something from someone in reshut hayachid and giving it to someone in reshut harabim (or vice versa).

Transferring from reshut hayachid to another via makom petur.