dtryon wrote:
"I assumed that entering 2 space 6 or 2.6 would display as 2h 36m."
Your assumption was incorrect. 2 6 is not recognised by Numbers as a duration value, and willl be treated as a text string. 2.6 will be recognised as a number, but not as a duration value. 2h 36m is a duration value and is the safest format to use when entering durations.
2:36 is also recognized as a duration value, and is one of the duration display formats offered in Numbers. but that format is also used to express the 'time of day' part of a Date and Time value, making it a less safe format to use to enter duration values expecting them to be recognized as such.
"No matter which I use even with the field set to duration it reverts to automatic."
This is expected behaviour in Numbers—if you enter a value incompatible with the accepted formats of the type of value the cell is set to expect, the data format setting of the cell will revert to Automatic, and the value will be accepted and displayed using the value type Numbers expects to receive in that format.
"Then what is the purpose of the custom format? I could simply enter 2h 6m as text."
You could enter 2h 6m as text in a cell whose data format is set to text, and it would be treated as text in that cell. It's default alignment would be to the left side of the cell, it would not contain a duration value, and it would likely not be useable in arithmetic calculations. The purpose of the specified duration format is to ensure that Numbers recognizes and treats the 2h 6m entry as a duration of two hours and six minutes, and can use that value in calculations.
BTW, 0h 0m is a specified format for duration, included in the list of formats directly choosable in the Cell format inspector.
Custom formats are not on the list, but are created by the user, using the tool that opens when the user chooses Create Custom format… in the Data Format menu in the Inspector.
Regards,
Barry