Debentures are a form of loan capital that a company can issue to raise funds. Unlike shares, debentures do not represent ownership in the company. Instead, they are a type of bond that pays a fixed rate of interest to the debenture holders. The company is legally obligated to repay the principal amount borrowed when the debentures mature.
In summary, debentures are not part of a company's authorized share capital, issued share capital, paid-up capital, called-up capital. Instead, they are a type of loan capital that a company can issue to raise funds.