I have custom post type called 'Porjects', and I have second custom post type called 'Clients'. Now, I want to 'Clients' be category for 'Projects'. How can I do this? Clients is custom post type not taxonomy.
I did the exact same thing (connecting projects & clients) using the Advanced Custom Fields plugin. I did this because my clients needed custom fields as well (logo, website URL).
I used the 'Post object' field for my project page, which will create a dropdown of your clients. Some editing of your template is required (to show your client info near a project), but the documentation on the plugin's site is really good.