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Thank you very much, Sarah, for the suggestion of "ad hoc" which helped to get me unstuck. Before asking my question, I had been toying with "As needed", but that didn't seem to capture the full sense and sounded a bit ordinary. In the end I used "By ad hoc arrangement" so as not to muddy the distinction with the second scenario, which I translated as "As a partner". CMJ_Trans, I did like "one-off venture", too, but felt that "venture" had a flavor of "investment" which I didn't want to presume in this case. 3 KudoZ points were awarded for this answer
Sorry! It was so late when I submitted this I didn't realize that more context might be needed. The indiustry is telecom/IT and the issue is how a company will offer information and system integration services. There are three scenarios. In the second scenario, it is through partnering. In the third scenario, the company has the necessary capabilities in-house.