In AXI3 and AXI4 protocol document, the recommended transaction ID up to four bits in master components. Also, up to four additional bits of transaction ID for master port numbers inthe interconnect is recommended. How the number 4 is chosen? Does it have any special meaning?
I don't think it has any real significance other than 4 bits supporting 16 encodings.
So if you had to define a default width, having support for up to 16 managers, each capable of issuing up to 16 different transaction IDs, seems to be a reasonable default value.
But it is only a suggested default width, only to be used where you don't have a definite view of what your system needs. Any values smaller or greater than 4 bits should be used if those are what your system does actually require.
Thank you for your reply!