When modifications are made to rows in temporal tables, Teradata Database provides automatic handling of the valid-time periods as needed to maintain the information in a time-aware fashion.
DELETE and UPDATE modifications to valid-time tables can use the FOR PORTION OF qualifier to place time bounds on the effectiveness of the modifications. The time during which a change is effective is called the period of applicability (PA) of the modification. Teradata Database selects the rows that are affected based on the relationship between the PA of the change and the valid-time period of the row, also called the period of validity (PV) of the row because that period defines when the information is considered to be in effect.
The relationship between the PA of a modification and PV of each row determines which rows qualify for the modification. It also determines when the change is effective and whether new rows will be automatically created in the table to account for data that changes during the valid-time period.
The mechanism of new rows being automatically generated by Teradata Database for modifications to temporal tables is described in more detail in Modifying Temporal Tables.