Learn English – Single word for company’s internal solution

If you want a single word that can make up a section heading, then Clients or perhaps even better External clients is probably a good fit. "Client" is a reasonably well-understood term to mean "someone who is the final receiver of the product created", but unlike Lascivious Grace's suggestion of freelancing need not imply any particular form of employment.

However, I'm wondering if it might not perhaps be even better to put this into your regular employment history section, rather than as a section of its own? In which case, you could write something like (note that this uses the same word, just not as a section heading):

2009 -- 2013, Acme Designs, Corp., graphics designer and web developer, with clients including Coyote Hospital, Inc. (2010 -- 2013); Funny Movies, Corp. (2009); and Acme Explosives, LLC. (2009 -- 2011)

You could then break that up (bullet point list, newlines, ...) however you feel fits your resume's style best.

If something is internal, it's obviously bounded by the limits of whatever it's contained within (a house, your body, etc.). If it's external it must be outside those limits (it's not bounded). So...

boundedness - the quality or state of being bounded (Merriam-Webster)

For an obviously "boolean" attribute like this, it's also perfectly natural to simply use a word identifying one of the two possible states. Just as we can refer to the truth of a statement (an "attribute" whose value can be either true or false), so we could refer to the...

internality - the quality or fact of being internal (in various senses) (OED)

...where the possible values of the boolean attribute are internal and external.