As stated earlier, SharePoint Content Types (including ECTs) represent entities and encapsulate all the necessary metadata about those entities. BDC Models represent a collection of ECTs and any relationships between those ECTs. So if you wanted to represent publishers and books and associate all the books published by each publisher, you would need to define the publishers, books, and their association within one BDC Model. BDC Models can be implemented in code via Visual Studio (VS) or can be implemented using XML via VS, SPD, or notepad. If they are implemented using XML, then the resulting file is referred to as the BDC Model (.bdcm).
BDC Models could include other information such as permissions for the ECTs, system or line of business properties, localized names, or proxies, but these types of information are considered resources and are best either implemented in Central Admin or placed in BCS Resource files (.bdcr). These can also be implemented via code in VS, but it is not usually a good practice.
Now although you can create a BDC Model, that doesn’t mean that you can search that model in SharePoint Enterprise (or FAST) Search just yet. You will have to mark the Model and each ECT as searchable. You will also need to create a profile page for at least each model and may create a profile page for each ECT. It is important to note that none of the steps necessary to make your solution searchable can be implemented directly in SPD, you will either have to open your model in notepad or VS and make XML changes for this to work. I will go into further detail about exactly how to do this in a future blog.