I am a novice to Confidence intervals. To figure out the confidence interval for mean, one could either use the $Z$ distribution or t distribution depending on the sample size and population standard deviation. When the size is less than $30$ and standard deviation is unknown, we go for t distribution. On the other hand, when the standard deviation is known, we go for $Z$ distribution.
Confidence intervals for mean is used to quantify the uncertainty by providing a lower limit and upper limit that represent a range of values that will represent the true population mean with a specified level of confidence.
Now, in the case of $Z$ distribution, how is the population standard deviation alone known prior to the estimation of population mean? Or in other words, what are the cases when population standard deviation is known before estimating the population mean?