Have you seen DISC profiles for sale that seem different from each other? Is the difference more than a lowercase i? Yes. It’s important to know which assessment is actually under discussion. They are not all the same. When you see claims that millions of people have used DISC, don’t assume that millions have used the version you might be discussing or considering buying. Over one million people a year use an Everything DiSC assessment.
The various DISC tests vary greatly. The colors used, the survey instrument, and the reports all differ. And some providers of free versions will retain respondents’ email addresses for marketing purposes. Only Everything DiSC has the added measurement for priorities, which you’ll notice are reflected in the words around the outside of its circumplex map.
The DISC model is not protected by copyright. William Moulton Marston did not create an assessment tool himself, but others have referenced his model. Several academics and companies have used his work to create their own psychological inventories.
I can tell you why and when there's a lowercase i in DiSC. It's part of Wiley's registered trademark for their DiSC Classic and Everything DiSC profiles. Not every assessment or test based on the DISC model is the same. That's why it's very easy to get different styles when taking different varieties of tests. Everything DiSC is norms their scale so a roughly even number of people who speak English (or another language the profile is in) get scored in each of the four main styles. Other forms of DISC put most people in the S category. The free versions have no research backing for them at all. I always recommend Everything DiSC Workplace to people who want something that's validated and reliable.