Welcome to the Bunnings Workshop community @kathyht. It's great to have you join us and many thanks for your question.
I'll need to contact Ezylite on Monday to get you the correct information and I'll let you know as soon as I do. There is, unfortunately, no industry standard for whether the dimples face upwards or downwards that we can rely on. It appears that the diffuser will work effectively in either orientation and is purely the manufacturer's preference.
I'll reply here when I can make contact with the manufacturer to see if there is a specific orientation which they require.
I've just made contact with Ezylite and their diffuser goes dimple side down.
Please let me know if you need further assistance or had any questions.