Unsearchable field?

Summary:

I created a column (Customized ID = calculated field from ID) to be a reference between two tables - master details tables. but it shows this field is unsearchable.

Content (required):

Hi Experts,

Master Table: BO has a default ID. but I need to customize an ID. So, I chosed to create a field as calculated field from the default ID.

Details Table: I am going to use the calculated field as reference to the details table. but when I choosed the field, I got a message that it is unsearchable field.

what does that mean?

I need it to be searchable. So, my user can search with this calculated ID.