Links field are the ones linked to another document type. For example, customer field is a link field in Sales Order. This field is linked to the Customer master.
You can insert Custom Link Field by following steps below.
####Step 1: Go to Customize Form
`Setup > Customize > Customize Form`
####Step 2: Select Form
In Customize Form, select Document Type (Quotation, Sales Order, Purchase Invoice Item etc.). Once field are updated in table, open field before which you wish to insert Custom Field. Then click on "Insert Above" to insert new Custom Field.