Create Subheads in Numbers

If what you posted in your screenshot is what you are talking about, it appears you have already created what you wanted. I assume it is not, though.

SGIII's screenshot shows the "spreadsheet" way to do it. You can then turn on Categories, with a category for column B.

Original table:

Categorized on "room":

Hide column B:

You can collapse/expand each room, or all rooms at once, by clicking the disclosure triangles. Option click on one will affect them all.

You can also do sub-categories if you have some other columns to categorize by

And you can drag the categories to re-order them

Could you explain what you mean by "subhead"?

A Numbers table can have up to 5 Header Rows and 5 Header Columns.

General information about tables in Numbers can be found here:

Add or remove rows and columns in Numbers on Mac - Apple Support

SG

Not sure what to call that. It seems like something you would more commonly see in a word processing document (say a document in Pages) than in spreadsheets. Perhaps Pages is more suited to what you are trying to do?

In a Numbers table you would more typically see things in tabular format, something like this:

A tabular format allows you to easily summarize data in various ways.

If you are new to Numbers I highly recommend having a look at the templates at File > New in your menu and at the User Guide at Help > Numbers Help in your menu.

SG