The transport table is for postfix transports. See postfix manpage for details for what transports are used for.
Well, I know very well that is for postfix and I plan to use one, but when or how ISPConfig uses this table
Postfix uses this table, not ispconfig. ISPConfig offers you to create transports like it offers you to create mailboxes, email domains etc. If you know what transports are, then you should know already what you can do with that table.
