Oracle Database Discussions

hi guys,

i read that a oracle_home + oracle_sid uniquely identifies an instance.
i have also read that a oracle_home can run different instances/pfile at different time.

so does that means that, since oracle_home+oracle_sid identify a instance, if i run an instance from a specify oracle_home (e.g. lets say oracleHome1 ),
i can only identify/connect the instance correctly if i connect to it from oracleHome1 ?

for example,
step 1) i run sqlplus.exe from oracleHome1. i start the instance(pfile) from oracleHome1.

step2) then i run sqlplus from oracleHome2 this time, and i have got my environment variable set to oracleHome2 , it means i cannot connect to the instance this time ?

if that is the case,
why at the client side when i am connecting to my db server, my oracle_home at client is totally different from the oracle_home at the server, and yet i can still connect to it ?

or does it means that
1) connecting to the db is 1 issue - any oracle_home will do
2) setting the oracle_home is another issue . as long as the oracle_home environment variable is set correctly to that of the oracle_home the instance is running under, connecting from any oracle_home is of no issue.

would someone just shed some light on me, i know my concept is wrong.. but please do bear with me and if possible explain in some very dummies terms as i am very new to oracle.

thanks!