A big role in this matter is the purpose of acquiring BTC. If, for example, you bought bitcoin in order to further benefit from the sale at a higher price, then this is an investment. If you bought bitcoin for what would be with someone to pay them for a product or service, it is not an investment but a payment instrument. For example, gold is now usually an investment, but more recently, coins were minted from it and used as a payment means and then it was not an investment.