Sometimes you may see some fields in grey color and you can't create a link while setting up a link & load, connection. The reason would most likely be two reasons explained as below:
This type of link will not be permitted in the link manager, as you cannot link from a subtable and load to a regular field (from a subtable on the right side of the link manager to the left side), since a subtable could have many different values entered in an entry, but a regular field would only have one value.
On the contrary, you can link & load from regular fields to a subtable, this works best if you would like to manually select (or type to see options) one field of the data, and load the corresponding information in other fields within a row of the subtable, such as having multiple line items for products in a sales order.
The linked field is a kind of selection field (select from other sheet). In order to identify each selection as unique, the linked field should be linked from a key value that can identify each entry on the source sheet as a unique entry, which is usually the id number or serial number field. Thus, you cannot link from a linked field or a numeric field as the linked field. However, you can link from a linked field or numeric field as a loaded field.