Here is an overview of the Catalog Manager Migration Tool steps:
Customize fields for items or tables
Catalog Manager does not recognize Custom Properties associated with specific items (on the object level). When you built your Store in the Editor, you set up custom properties (unique fields) for specific items. In the new Catalog Manager, you can apply a custom field to all the items in a given table but not to individual items. (In a table, a series of similar items, such as toys or T-shirts, are gathered together in a product category.)
You must choose whether to preserve Custom Properties for specific items by maintaining them in the Store Editor or whether to migrate the item to the Catalog Manager and convert the Custom Property to a new custom field for an entire Catalog Manager table.
Resolve Custom Property naming conflict
This step occurs when you have identical custom property names (unique fields you created when building your Store) for different data types. Although the Store Editor allows you to have Custom Properties for items where the Property Name is the same but the data types are different, this is not allowed within Catalog Manager.
To resolve this conflict of mismatched data types, please decide whether to leave these properties in the Store Editor or migrate them to the Catalog Manager. If you migrate the custom property to the table function in Catalog Manager, you must rename it as well.
Resolve Custom Property/System Field naming conflict
In this case, you are required to rename a specific Custom Property so that it doesn't conflict with the name and associated data type within Catalog Manager's built-in, optional Store or Shopping fields.
Mandatory fields hold common information about your items, including name, price, and ship weight, and must be provided for your items. Store fields are used at the product category (table) level to allow you to populate an entire product category with relevant data for specific types of merchandise. Shopping fields are essential for merchants who participate in Yahoo! Shopping. This rename is required.