let me try to answer all of your question
1/ In the Used(Host) column, there are some rows have no information like CN.DB.DB2? this means the license is not yet used? Why in other rows, we also have 0 used?==> CN.DB.** is the license for AE database connection so right now we only support ORACLE, MSSQL, and DB2, If you want to run the Automic System based to another Database than MS SQL you require a specific license entry (CN.DB.ORA or CN.DB.DB2) in your license, for MS SQL it is not required. and "used host" is not applicable for this type of license
2/ For the SAP, we have 999999, unlimited license?
=> for SAP agent for Basis you have 999999 so kind of unlimited
3/ About EX.OS.WIN (S and V class), it means that I have 4 licenses for Windows? I can install 2 more agents on 2 windows machines?=> Class S only allow you install the window agent on the same server with the Automation Engine (where we run the CPs or WPs),
class V you can install the Agent any where, And yes you have 2 more license
4/ What are the purpose of DC.WEB, Company and GL.DC?=>
DC.WEB is for Web Interface , I believe this one is not being used anymore (Starting from V10 we removed the web interface, noted it's difference with ECC/AWI)
5/ About EX.OS.OS4 is categorized as Test, we need to purchase license for Test?=> I can see the license you have for category "Test" is expired, so you will not be able to use that license in the existing AE system,
Let say if you want to install one more OS400 agent for testing only then you would need to have the valid license. (Test Category)
6/ I don't find info the FTP in the document, I guess it is used for ftp connection.==> go to this
link for your information about FTP agent