The main piece of information a tag is transmitting is its unique ID (or serial number). This is in the form of a long (many digits) number, having a length is usually expressed in bits.
As rough comparison, every 10 bits represents three digits when the number is expressed in decimal format. A 266 bits long ID would have a range of values comparable with the numbers of atoms in the universe (1080).
While the number is called "unique ID", for writable technologies it is not guaranteed to be unique - nothing prevent producing two tags with the same ID. For read only technologies, the fact the number is unique is dependent on the lenght of the ID, but for most applications even relatively short IDs can be considered "locally" unique.
When the serial number is writable, it is good practice (also standardised by GS1 for the Gen2 tags) to adopt a structured format.
