A platform build version is the specific compiled release of the trading software running on a machine, and it is not the same thing as the broker's server version. Both are numbered independently, both are updated on their own schedule, and behaviour a reader observes is a property of the pair rather than of the platform in the abstract.
A mismatch is usually not silent for long: some servers refuse a login from a terminal older than they will accept and push an update instead. Quieter is the case where the terminal connects but a feature behaves differently — how an order type is handled, what a programming interface exposes, which settings exist at all. That is what makes a build number worth recording before anything else in a bug report, and what makes a screenshot or a written instruction go stale without anyone noticing: the menu it describes may simply not be in the build the reader has.
Next step
Guide
Choosing a Trading Platform: MT4, MT5, cTrader and the Broker's Own
The platform decides which orders you can place, whether your stops survive a closed laptop, and how much you would have to rewrite to leave. What actually differs between the options — and the point where the platform stops mattering and the broker takes over.