Some decisions of the general meeting of shareholders are made not by simple majority, but by qualified majority — for example, charter amendments, reorganization, major transactions. The owner of a sufficiently large stake cannot push through their own decision, but is able to block someone else's.
The classic benchmark is a stake exceeding one-quarter of voting shares, since a qualified majority typically constitutes three-quarters. Specific thresholds and the list of matters requiring such majority are set by the law on joint-stock companies and the charter of a particular company, so they should be verified in each case.
For a minority investor, the structure of stakes is more important than their names. If one person holds more than three-quarters of shares, other shareholders lose the ability to influence key decisions altogether. Conversely, the presence of an independent holder of a blocking stake usually means stricter control over management actions.