Agree one topic scheme per organisation before devices multiply, and put site, asset and measurement in the same order every time.
MQTT defines transport, not meaning
MQTT moves messages to named topics through a broker. It does not say what a topic should be called or what the payload contains. Two vendors can both speak MQTT and still be incompatible.
A simple convention
Use a path that runs from broad to specific: site, then area, then asset, then measurement. For example, factory/line-2/oven/temperature. Keep names lowercase, avoid spaces, and never start a topic with a slash.
What to avoid
Do not put values or timestamps in the topic. Do not use one topic for many measurements. Do not let each team invent its own scheme, because wildcards then stop being useful.
QoS and retained messages
QoS 0 may lose messages, QoS 1 delivers at least once and QoS 2 exactly once at higher cost. Most telemetry uses QoS 0 or 1. A retained message is delivered to new subscribers immediately, which can make a dashboard show an old value as if it were live unless you also track message age.
Security
Use TLS on port 8883, authenticate every client and limit each client to the topics it needs. An open broker with wildcard access exposes every device behind it.