hope everyone is well during these difficult times we’re going through.
I have a logo which I want to map onto my model. I have marked the seams, even separated the mesh from the model, made a material, just for the specific location which I want to map my logo. But when I do the UV mapping, the UI text is getting replaced by the logo. What am I doing wrong?
Thank you.
You have two UV Maps, one is fine as you want to and the other has the logo. The logo must have transparent background so it can be masked later. The trick here is to project from view the faces you want the logo to appear (no perspective enabled). The rest of the faces can be “parked” in some transparent area and get them out of the way.
Then you can combine the two textures in any way you want, either as colors or materials. The color inputs however are a bit tricky though, you will have to make some trial and errors until you get it right, is somehow difficult to remember exactly.
Though I got texture repetition on the sides, for that fix I would expand the logo boundaries so the entire UV projection can fit all inside the texture space (texture repeat is caused by UVs going out of bounds actually). If someone knows a quick fix to clip the texture it will be good know, refer to me with @ and username to let me know. 
A few observations…A UV sphere already has UV, so selecting an area on the sphere will show up in the UV window… I get what you are trying to do… In your case with the UV…anything outside of the UV-space will repeat…you don’t need a second UV ( but that is one way to go) another way is to just select the faces you want the Logo to GO and assign another texture to it… and add the Logo as an Alpha Mask… Something like this…
Sorry I was working on this while others were posting…Might be repeating some things…
As HISEROD mentioned enabling this “Clip” setting in the texture will be good idea. Also this mask idea is good to achieve more complex effects by combining various patterns together.
Usually the Clip Blend works best if used with an Alpha Mask ( B/W ) plugged into the Mix shader as a Factor… If just used with out , many times it will cause the Mesh to be transparent.
Used extensively for Hair and Fur in Game shaders…
