I am struggling to understand the difference between these two. They seem to mean the same thing and are used interchangeably.
Investopedia also has two different article on them.
Paid-in capital is the amount of capital "paid in" by investors during common or preferred stock issuances, including the par value of the shares themselves plus amounts in excess of par value. Paid-in capital represents the funds raised by the business through selling its equity and not from ongoing business operations.
Paid-up capital is the amount of money a company has received from shareholders in exchange for shares of stock. Paid-up capital is created when a company sells its shares on the primary market directly to investors, usually through an initial public offering (IPO).