header: identifier for the custom data. If you plan to add custom data (as a company or individual), always use the same header (because only you will know what data type is stored under that header) and stick to it. The best is to use the serial number of your V-REP copy (check the "Help" menu, in the "About" item for the serial number). Otherwise, you risk collision with other developer's data which might use the same header as yours.
data: your custom data. If NULL, the current data under that header will be removed. If you have several items or data types to save, it is your responsibility to pack and code it in data (don't use various headers for each of your items that you want to save with a scene (risk of collision with other developers data (see above))!)
dataLength: the length of your custom data
The data will be copied to an internal buffer, and next time a scene is saved, will also be saved. The data buffer can be released after this call.