'Sirenix Box' is a box that a Pre-Sirenix fairy earns during her quest to become a full pledged Sirenix fairy. This box is where the Sirenix Guardians are hidden. This box also serves as the container of the Gem of Courage, Empathy, and Self-Confidence.
The Sirenix Box is a box recquired for a fairy to collect as a stepping stone to become a full Sirenix fairy.
The Sirenix Box appeared frequently in Season 5. Since the Winx has no knowledge about Sirenix and they need to ask their guardians, they often call these boxes for help.
When the Winx found the Sirenix Book, in the first time they attempt to read it, it won't open at all. The Sirenix Book only opens when it has something to say. It never really does open when it's not necessary at all.
In order to start the journey to become Sirenix Fairy, the winx must find the Rainbow Mantle first, and then after that, they need to find the three gems called the Gem of Empathy, Self-Confidence, and Courage. They need to do it all before a lunar cycle ends. If they fail to find all of it after the lunar cycle ends, they will have to kiss their powers goodbye.
After the Winx went to the Source of Sirenix and used their Sirenix Boxes to get the transformation, these boxes were no longer necessary.