Topic is like a staging table structure which DIH uses to refer the structure of the source table. Publication workflow will pick up the data from source and places into the staging table from where subscription will read data.
The Video KB describes about the three different methods to create a topic.
1. From database - we can connect manually to the database and select the source table, so that DIH will create a staging table in the staging repository. DIH will append two columns in the staging table which tracks the publication id and publication date.
2. Manually - we can create staging tables manually and DIH will create those tables and add two columns for publication id and publication date.
3. From workflow definition file - we need to import the workflow from PowerCenter repository manager and upload the workflow into the DIH console. DIH will parse the target table and create a staging table in the staging repository. In this case we need to manually add two columns - for publication instance id and publication date.