I use "db2 get snapshot for all on zmccdb" to get the following information;
Database name = ZMCCDB
Database path = /home/zmccinst/zmccdb/db2inst2/NODE0000/SQL00001/
Input database alias = ZMCCDB
Database status = Active
Catalog database partition number = 0
Catalog network node name =
Operating system running at database server= AIX
Location of the database = Local
First database connect timestamp = 07-13-2004 21:00:52.200086
Last reset timestamp =
Last backup timestamp = 08-01-2004 21:00:42.086460
Snapshot timestamp = 08-02-2004 17:52:43.010177

High water mark for connections = 21
Application connects = 1331
Secondary connects total = 0
Applications connected currently = 7
Appls. executing in db manager currently = 0
Agents associated with applications = 7
Maximum agents associated with applications= 21
Maximum coordinating agents = 21

I want to know that the meanings of "Application connects" and "High water mark for connections " and "Applications connected currently".
Also, I want to know how to get the session number of DB2 just like oracle.
Thanks!

'list applications' will give you all sessions with their respective application handles (an integer).

With 'get snapshot for application applid xxx' you get all details on the connection (including the identity from the client)

If you are on Version 8, you can use the snapshot table functions snapshot_appl_info and snapshot_appl ..
Eg:
db2 "select * from table(snapshot_appl_info('SAMPLE',-1)) a"
db2 "select * from table(snapshot_appl('SAMPLE',-1)) a"