This only applies to DOS mode partition tables which are being phased out with the more modern GPT partition tables. However with DOS partition tables a physical harddrive could only have up to 4 Primary Partitions.
If more partitions were needed, the first 3 partitions could be set as Primary, with the 4th being an Extended Partition. Â The 4th partition could now have within it up to 63 Logical Drives (acting just like Primary Partitions).
The only catch with Logical Drives is that they couldn’t be used to start an Operating System.