That seems really odd; an asset custom field shouldn't be showing up in the ticket submission. Perhaps a screenshot or two would help illustrate the issue?
If you're under maintenance i would likely open a support ticket on that as it doesn't sound right.
Yes, maybe I didn't explain this very well. So I am going...
Step 1 - Click on New Ticket
Step 2 - Lookup & choose the client on the Client Info tab
Step 3 - Click on the Asset Info tab, which shows their assigned assets. It is on this page I am seeing an Asset Custom field which is not appropriate for the asset type (shown in the screenshot, Machine Type (IBM)).
If the Client has no assigned assets & I look up an asset it is the same, the unwanted Custom Field is showing.
If you think this is a defect rather than how I have set it up I will log a ticket...
That does seem like it may be a bug.. i'd open a ticket.
Ok, I think I have figured this out but to be honest the answer doesn't make much sense to me!
Some time after creating the custom field I had also created a new Column Set in the Asset Search section which included this field. It looks like this extra column was being included for every asset in the Asset info tab i.e it must modify the default search somewhere & pull this column in too. Weird but at least I know how to sort it out