U is an internal Universa measurement unit of energy required to execute some Universa transaction, i.e. to register some smart contracts/contract revisions.
The simplest and lightest transactions usually take just 1 U to be registered, more complex ones may require more. The details of calculation how much Us some transaction requires, are rather complex and low-level (you can find them at Quantization article in the Software Developer Central); but most of the times you just need to top-up your U balance (“reserve some U”) using either UTNP or UTN tokens.
Typically, the price of 1 U is around €0.01.
U smart contract:
- is not a token; not intended to be used for any payment means. It cannot even be combined with other U token, cannot be “sent out” – the only supported method of operation is spending (i.e. decreasing the amount), not sending.
- does not even supports changing the owner. The other reason why you cannot send it to someone and use it as a token, is that the owner of U smart contract is fixed at the moment of receiving the U contracts.
- is time-limited. At the time of writing the article (early 2019), the lifetime of any issued U contract is set to 1 year. After 1 year passed since issuing the contract, you should expect the contract becoming invalid and REVOKED/UNDEFINED.
- U stock in Web Client, and how to top-up it.