@Dan ******
yep, you'll need to show evidence and explain that because they'll ask for company/business registration if it's not clear to them.
Australia it falls under the personal income/sole trader area to buy/sell shares under own name. So more the entrepeneur/free lancer but you're not providing services like most businesses do and don't need to produce invoices or receipts to customers. It still requires having to document it like a business for when you submit your taxes.
I'm not sure how investing under own name works for taxes/legal obligations in America.
I was asked for company and business registration which I also don't have and had to provide details about why.
The people processing the applications are making sure that you are able to earn an income generated from outside of Thailand to support yourself, that it's nothing dodgy, and don't have the same knowledge as you about it.