To answer your question, I will first give a very brief description of the difference between ANT, ANT+, and FIT.
ANT is the wireless protocol that defines the wireless connection between the devices.
ANT+ devices all use ANT to establish the wireless connection. ANT+ device profiles specify the ANT channel parameters and data content to ensure interoperability between ANT+ devices.
FIT is currently a file format that both ANT+ and non-ANT+ devices can use to store data. Some ANT+ devices can store and transfer FIT files, others only broadcast data. The broadcast data format is defined in the relevant ANT+ device profile, not by FIT.
The only requirement for devices to be able to interpret each other's FIT data is that they have the same "product" profile... The FIT SDK contains all current, publicly defined, FIT messages. We can call this the "Global Profile"
One developer may choose to create a "Product Profile" that is a reduced version of the SDK... for example, to use only HR related messages and fields... In this case, if the product received a FIT file containing both HR and bicycle power data, then only the HR data could be interpreted as the bicycle power definitions are not part of the implemented "product profile"...
Similarly, the FIT protocol does allow for manufacturer specific messages to be defined... and these are specific to the "product profile"... so if you want to create your own messages... you would need to give those definitions to the intended recipient such that they can decode them correctly.
If you have data that you would like to be interoperable and included in the FIT SDK, you can join the ANT+ Alliance and we can discuss how to proceed.
I hope this clears things up.
