ODS = On-Disk Structure.
ODS (on-disk structure) is a number representing version of the database physical layout structure. When a new feature is added to Firebird it might or might not require the structure of system tables (database metadata) to change. If it does, the ODS number must increase.
This number is checked upon connection, so that server makes sure it can 'understand' the database structure. For example, when you try to connect with a 1.0 server to a database created with Firebird 2.0, you'll get an error as the 1.0 server is not able to handle that ODS - simply because there are fields whose meaning it does not understand.
The ODS version shows with which database version the database was created, e.g.:
|Database created with version:||ODS version:|
|InterBase® 5.5 / 5.6||9.1|
|InterBase® 6 / Firebird 1.0||10.0|
|InterBase® 6.5 / Firebird 1.5||10.1|
|InterBase® 7 / Firebird 2.0||11|
|InterBase® 7.1 / Firebird 2.1||11.1|
|InterBase® 7.5 / Firebird 2.5||11.2|
|InterBase® 2007 / Firebird 3.0||12|
For more information about the InterBase® on-disk structure, please refer to Ann Harrison's article, Space Management in InterBase®.