#2 API tweak - "," vs "."

[Jim Jewett]

period and comma are both used to separate keys.
Please clarify how/why? How are they joined, is there
any difference between a period and a comma?

For instance, does "key1.key2,key3" mean "Find it in
section key1, subsection key2. If it isn't there, find it
in section key3"?