Testing the software build wether it is coexisting with the 
other softwares to share the common resources is called 
Intersystem testing
say for example,In e savas there will be current bill 
section ,water bill sactions both the software sharing 
common data from the database servers

It means that,sharing the resources of one system with
another.Eg:Suppose u have icici bank debit
card.OK.unfortunately u need a cash very urgent,u found sbi
atm nearby u.OK.U just and insert icici card and enter ur
pin.OK.

what the process will happen is.

The request u made is first it sents to sbibank server and
then transfers to icicibank server and process will do in
iciciserver and again it sents to sbibank server.now u can
get cash from atm.The conclusion is sbibank server is
sharing the resources with icici bank server and performing
the required operations.

Intersystem testing is the process in which wheather the one
system of resources is sharing with another system resources
or not

I hope u clear

Test the application to ensure interconnection between 
application functions correctly. It does:
- Determine Proper parameters and data are correctly passed 
between the applications 
- Documentation for involved system is correct and 
accurate. 
- Ensure Proper timing and coordination of functions exists 
between the application system.