What does "enclosed" on the invoice mean, is it for remittance? Can I change it from showing up or use that field for something else?

"Enclosed" traditionally, pre-digital world, means that something else besides the document is in the envelope you just opened.

You might be able to edit the template to change the label for this text field

Thanks for joining the Community, tammy47. I can share some information about what this term means on emailed documents.
 

When you send an invoice to a customer, they'll see a message that says "Enclosed". The reason for this is so that it's clear there's an invoice attached to their email. It drives your recipient's attention toward their document.
 

Currently, there isn't an ability to customize and remove this line from outgoing emails. There might be some thirty-party applications which can help change it in certain ways. You can search our QuickBooks App Store for apps that may be able to accomplish such a thing.
 

As a QuickBooks Online subscriber, your voice definitely matters. If you'd like to share this idea with Intuit's product development team as a feature request, you can do so while signed in:
1. Click your Gear (⚙) icon.
2. Select Feedback under the PROFILE section.
3. Once you've entered your recommendations, hit Next.

Submitting feedback helps Intuit understand where customers want products to be improved and what features they're interested in.

I've included an official resource about customizing invoices, which will be helpful in understanding how they can presently be customized: Customize sales forms in QBO
 

Please feel more than welcome to send a response if there's any other questions. I'll be here to assist. Have a wonderful day!