What is the highest cloud in the sky?

Could you please explain to me, in a clear and concise manner, what exactly is meant by the term 'the highest cloud in the sky'? Is it referring to a specific type of cloud, or is it a more generalized concept used to describe the uppermost limit of clouds in our atmosphere? If it's the latter, what factors contribute to determining this height, and how does it vary depending on location, weather conditions, and other environmental variables? I'm particularly interested in understanding the scientific principles behind this phenomenon and how it relates to the broader field of meteorology.