LoadRunner addresses this issue through correlation.
Correlation saves the changing values, in our case the
session ID, to a parameter. When running the emulation, the
Vuser does not use the recorded value—instead, it uses the
new session ID, assigned to it by the server.
Correlation is used to obtain data which are unique for each
run of the script and which are generated by nested queries.
Correlation provides the value to avoid errors arising out
of duplicate values and also optimizing the code (to avoid
nested queries). Automatic correlation is where we set some
rules for correlation. It can be application server
specific. Here values are replaced by data which are created
by these rules. In manual correlation, the value we want to
correlate is scanned and create correlation is used to
correlate.

Property wise both are same, the difference lies in the 
understanding, in automatic correlation there is some set 
of rules and we have to import these rules for specific 
protocol and it is done automatically by the rules just we 
have to click options before recording, while manually we 
search for the possible dynamic value generated and we 
apply function to capture the values web_reg_save_param and 
place at the proper place.