Key to use "value_conv" field in ModBUS mapping profile

[PrimusIP]

Hi everyone! I'm just starting in LM->ModBus and in JSON mapping I was found field "value_conv". Can you help me to understand how I can use it and give some examples?
So here http://www.openrb.com/docs/modbus.htmvery little information to understand exactly how it works.

[admin]

These are internal functions which are not usually needed in custom profiles.

[PrimusIP]

Yes, I understand this, but I just want to know where, when and how I can use it ). I can't find it information anywhere.