For the APIs, Channable has an 'Attribute builder' phase. The Attribute builder is a convenient feature that gives you a clear overview of the required attributes per category for that API . This is especially useful if you sell several different kinds of products that have very specific attribute requirements.
In the attribute builder tab you see an overview of every category to which you have assigned products. In the column to the right you see how many products are in each category. The categories in this list are sorted by the number of products that are assigned to them. This way, you can prioritize large categories that would be time-consuming to do manually.
Once you've selected a category, you will see an overview of all products in that category. Fields that are marked in red are required but missing in your data. You can click any column label that has the pencil logo in it.
This will open a window in which you can enter an attribute value. You can enter a value as plain-text by simply typing in the box. This value will then be the same for each product in all categories. Alternatively, you can click the plus button and pick an attribute.
This way you can pick any field you like. This field applies to all products in all categories as well: the difference is that a field value can be different for each product. The Attribute Builder comes after the Rules phase, so you can modify and add any field data to your heart's content.
Aside from using either fields or plain text, you can combine either of the two. While this is not very common, it can be useful if you for example need to add text before or after a field value and don't want to use a rule. You can also combine multiple fields.
Once you're done picking your field, you can hit save and you will see your changes right away. If you don't see anything happening, make sure the field you selected is not empty.