As from the 3rd of January 2018, in line with the provisions of the second Markets in Financial Instruments Directive (MiFID II), prices on equities (shares) and exchange-traded funds for limit orders are to be entered in accordance with the applicable decimal price increments (tick size) for the respective financial instrument.
A tick size is the minimum upward or downward price movement of a security, and is determined by the stock exchange where the security is traded. For example, if the tick price of a security is EUR 0.01, when submitting a limit order on that security, the limit price that you enter must be a multiple of EUR 0.01 as the price of the security can only vary by EUR 0.01. Otherwise your order cannot be executed.
You can check the tick sizes of most frequently traded stocks on Euronext by clicking on this link.