Explained layers of the startup solution.

docs/en/Domain-Driven-Design-Implementation-Guide.md

@@ -108,13 +108,36 @@ The Application Layer is also splitted into two projects;
 
 #### The Presentation Layer
 
-`IssueTracking.Web` is an ASP.NET Core MVC / Razor Pages application for this example.
+* `IssueTracking.Web` is an ASP.NET Core MVC / Razor Pages application for this example.
 
 > ABP Framework also supports different kind of UI frameworks including [Angular](UI/Angular/Quick-Start.md) and [Blazor](UI/Blazor/Overall.md). In these cases, the `IssueTracking.Web` doesn't exist in the solution. Instead, an `IssueTracking.HttpApi.Host` application will be in the solution to serve the HTTP APIs as a standalone endpoint to be consumed by the UI applications via HTTP API calls.
 
+#### The Remote Service Layer
+
+* `IssueTracking.HttpApi` project contains HTTP APIs defined by the solution. It typically contains MVC `Controller`s and related models, if available. So, you write your HTTP APIs in this project.
+
+> Most of the time, API Controllers are just wrappers around the Application Services to expose them to the remote clients. Since ABP Framework's [Automatic API Controller System](API/Auto-API-Controllers.md) **automatically configures and exposes your Application Services as API Controllers**, you typically don't create Controllers in this project. However, the startup solution includes it for the cases you need to manually create API controllers.
+
+* `IssueTracking.HttpApi.Client` project is useful when you have a C# application that needs to consume your HTTP APIs. Once the client application references this project, it can directly [inject](Dependency-Injection.md) & use the Application Services. This is possible by the help of the ABP Framework's [Dynamic C# Client API Proxies System](API/Dynamic-CSharp-API-Clients.md).
+
+> There is a Console Application in `test` folder the solution, named `IssueTracking.HttpApi.Client.ConsoleTestApp`. It simply uses the `IssueTracking.HttpApi.Client` project to consume the APIs exposed by the application. It is just a demo application and you can safely delete it. You can even delete the `IssueTracking.HttpApi.Client` project if you think that you don't need to them.
+
 #### The Infrastructure Layer
 
-TODO
+In a DDD implementation, you may have a single Infrastructure project to implement all the abstractions and integrations, or you may have different projects for each dependency.
+
+We suggest a balanced approach; Create separate projects for main infrastructure dependencies (like Entity Framework Core) and a common infrastructure project for other infrastructure.
+
+ABP's startup solution has two projects for the Entity Framework Core integration;
+
+* `IssueTracking.EntityFrameworkCore` is the essential integration package for the EF Core. Your application's `DbContext`, database mappings and other EF Core related stuff are located here.
+* `IssueTracking.EntityFrameworkCore.DbMigrations` is a special project to manage the Code First database migrations. There is a separate `DbContext` in this project to track the migrations. You typically don't touch this project much except you need to create a new database migration or add an [application module](Modules/Index.md) that has some database tables and naturally requires to create a new database migration.
+
+> You may wonder why there are two projects for the EF Core. It is mostly related to [modularity](Module-Development-Basics.md). Each module has its own independent `DbContext` and your application has also one `DbContext`. `DbMigrations` project contains a **union** of the modules to track and apply a **single migration path**. While most of the times you don't need to know it, you can see the [EF Core migrations](Entity-Framework-Core-Migrations.md) document for more information. 
+
+#### Other Projects
+
+There is one more project, `IssueTracking.DbMigrator`, that is a simple Console Application that **migrates** the database schema and **[seeds](Data-Seeding.md) the initial** data when you execute it. It is a useful **utility application** that you can use it in development as well as in production environment.
 
 ### Execution Flow a DDD Based Application