This is a short and quick post. If you have an entity and you want to add a link to the left hand side navigation menu of its form, then you can do this by one of two things:
1) Using ISV.CONFIG if this link is to an external website, application, etc.. This was discussed in more details in my post on using and editing ISV.CONFIG file
2) Creating a 1:N relationship between this entity and the entity you want to have a link for on the left menu bar.
So for example, if you want to have a link to all projects associated with an account in the account form, you will need to create a relationship between them. i.e. When you open an account form, you want to have a link to projects on the left handside navigation menu of the account form. To do this, you go to customisations, Account entity and in there, create a new 1:N relationship with your required entity (custom entity Project in this case). Create the relationship with Account as the primary entity and Project as the related entity. Save and publish the account form.
Now when you go and open any account record, you will see a link – submenu – item called projects on the left hand side of the Account form. If you click on it, it will allow you to add existing Projects (project records or whatever entity you selected) or you can create new project record.
You can of course apply this to any combination of entities whether customisable or custom.
To rename the left hand side menu navigation bar items, you can have a loo at my other two posts about renaming and hiding these items.