Русско-англо-русский юридический словарь

Back before we had electronic computers and the Internet, keeping track of shares was a lot of work. If a person had 1,000 shares then you might have to move around 1,000 printed share certificates. Therefore the practice developed of converting a person's shares in a company into stock. Stock representing 1,000 shares could be documented as a single item instead of 1,000 items. (Of course, you could still sell one 1,000th of that stock if you wanted to.) (StackExchange)

As per Section 61, Companies Act, 2013, the company can convert its shares which are fully paid up, into stock. A ‘Share‘ is the smallest unit into which the company’s capital is divided, representing the ownership of the shareholders in the company. A ‘Stock‘ on the other hand is a collection of shares of a member that are fully paid up. When shares are transformed into stock, the shareholder becomes a stockholder, who possess same right with respect to the dividend, as a shareholder possess. (Key Differences)

Stock is another form of share capital. A company cannot issue stock units but it can convert shares into stock. The stock units then represent a nominal amount of the share capital. For example, if £1 ordinary shares are converted to stock they will become £1 stock units. (GOV.UK)

One of the advantages of converting shares to stock used to be that shares had to be numbered but stock units did not. This made it considerably easier to effect transfers, particularly on the Stock Exchange, as the sale documents merely needed to refer to an amount of stock and not the numbers of particular shares. (GOV.UK)