Please can someone explain this to me

Any statements regarding your previous or future impact on the UK digital technology sector should be supported by relevant evidence where possible.

One of my future plans to starting my own business which I mentioned in my personal statement
Where else can I add it and the project is on development stage, I only have it on GitHub

Since it’s a project in a development phase, you can create one document showing its market disruption potential, jobs-creating potential, technological advancement potential, or potential social impact. You can use that document separately as one of 10 pieces of evidence you can attach and just mention in your personal statement to refer to that document.

Also, you can show if you have any design documents, prototypes, or demos. Also, any market research you did to see how your product will have a positive impact on the digital sector. You can also show if you have evidence of some early investors interested in your project, etc.

Okay, thanks @amm
One more thing.

Am I to add it on the mandatory criteria?

For this, I am not too sure so other experts on the forum might help you. I think mandatory and optional criteria are focused on the work you did in the past and not future but there is additional documents upload option as far as I remember where you can upload this and refer it in personal statement

Oh, there is additional documents option
Thanks for ur help