Pseudo-regular interval data is time-series data that usually has a regular-interval, but not always. For example, one may have daily measurements at a gage, but during an event, more measurements might be reported. Or, there might be "Local time series" data, where a measurement is taken every morning at 8:00 am, regardless if daylight savings time is in effect or not (meaning some days will be fore 8:00 am, others for 7:00 am).
data is actually stored using the irregular-interval time series conventions, except the "E part" looks like a regular-interval preceded by a tilde "~". The rest of the E part following the tilde should be the standard time interval that the data is usually at, for example "~1Hour".
Why do we have Pseuro-Regular data intervals? USACE districts have gages reporting every day at, for example, 8:00 am, then Daylight Savings time would come along and all of a sudden, there would be an hour difference, and did not fit the regular interval criteria. Pseudo gives each value it's own time stamp, so they could have their daily data with correct times and observe DST too. Pseudo is useful because there are a lot of gages that are "almost regular" and this takes care of those. However, most models don't like pseudo, only pure regular, so we usually need to convert pseudo to regular before running them.