Mains electricity in mainland Western Europe has been set at 220VAC 50Hz whereas the UK used 240VAC 50Hz.
Western European supplies are currently classified as 230VAC. In reality there is no 230VAC supply as 230VAC was a “standard” created during European "harmonisation" to give a single voltage standard across Western Europe, including UK and Irish Republic.
Ideally there would have been a single voltage however there were too many political, financial and technical obstacles to reduce UK voltage to European levels or to increase European voltage to UK levels, so a new standard was created to cover both. This was achieved by changing the tolerances of previously existing supply standards. UK voltage went to 240VAC + 6% and - 10% and European tolerances to 220VAC +10% and -6% (thereby creating a manageable overlap) and we would call these two combined 230VAC, despite the fact that nobody was intentionally generating at 230VAC!
Depending on the voltage sensitivity of the product and the variance from nominal of the actual supplied voltage, it may not be advisable to use a 220VAC specific device in the UK or a 240VAC specific device in Mainland Europe etc. For instance a 240VAC supply can rise to as high as 254.4VAC and still be within tolerance, but the maximum rated voltage for a 220VAC product is only 242VAC. A 220VAC supply can drop as low as 206.8 within tolerance but the minimum rated voltage for correct operation of a 240VAC product is 216VAC It may work perfectly well either way but it could be, technically, outside the specification of the equipment with obvious implications. A 230VAC product must be compatible with all voltages across this range
If a product is to be used in the UK a 240VAC rated device is ideal but either 240VAC or 230VAC products can be used and If a product is to be used in mainland Europe or Irish Republic a 220VAC rated device is ideal but either 220VAC or 230VAC products can be used.