What values are valid in Pandas 'Freq' tags?

You can find it called Offset Aliases:

A number of string aliases are given to useful common time series frequencies. We will refer to these aliases as offset aliases.

Alias    DescriptionB        business day frequencyC        custom business day frequencyD        calendar day frequencyW        weekly frequencyM        month end frequencySM       semi-month end frequency (15th and end of month)BM       business month end frequencyCBM      custom business month end frequencyMS       month start frequencySMS      semi-month start frequency (1st and 15th)BMS      business month start frequencyCBMS     custom business month start frequencyQ        quarter end frequencyBQ       business quarter end frequencyQS       quarter start frequencyBQS      business quarter start frequencyA, Y     year end frequencyBA, BY   business year end frequencyAS, YS   year start frequencyBAS, BYS business year start frequencyBH       business hour frequencyH        hourly frequencyT, min   minutely frequencyS        secondly frequencyL, ms    millisecondsU, us    microsecondsN        nanoseconds