How do you know if something is COTS?

Excuse me, but could you please elaborate on how one can determine whether something qualifies as COTS, or Commercial Off-The-Shelf? I understand that COTS products are typically readily available in the market, but what specific factors should one consider when making this assessment? Are there any defining characteristics or standards that universally apply, or does it vary depending on the industry or context? Additionally, how does one differentiate between COTS and other product categories, such as custom-built solutions or open-source alternatives? Your insights would be greatly appreciated.