Mapping of fields giving trouble

Mapping of fields giving trouble

Hi guys
 
I'm busy setting up my Zoho, having a lot of fun and excitement about implementing it.
Now, it seems not all fields, especially custom fields generated by me, are available to be mapped which doesn't seem to make sense.
I also don't know how Zoho decides to plug what field into the drop down menues available to be mapped under each category.
Is there a way to work on this?
I couldn't figure out how to move forward from here so my mapping right now doesn't make 100% sense, or is incomplete.
 
Please advise?
Thanks
Niels