How to customize the link field

I am facing an issue in customization of link field to fetch data.

  1. I have created a project id custom field as datatype (DATA) in projects module.
  2. I have linked the project id as data type of (LINK) in ITEM master module.
  3. Now problem is " while creating the purchase order I need project id which is created for item should be fetched automatically with that particular item.
    EG: If I create project id as ABC001 for the item 001 in Item master. While creating new Purchase Order When I am selecting the item 001 it should fetch the project id details in purchase order.
  4. Also how to create project id as multiselect option in Item master, While creating Purchase Order it should display the options of project ID created with that ITEM.
    EG: ITEM 0002 has project ID’s of A0002,B0002,C0002. This three project id should be display as option while creating a Purchase Order.
  5. How to fetch data from multiselect datatype or single linked datatype?

Kindly do the needful.