It’s too bad that when the subject of IJTAG vs. JTAG comes up, some may think of it as an ‘either/or’ question. That’s not really the point because the correct answer is ‘both.’
In a sense, we’d be better off if the IEEE 1687 standard for embedded instrumentation wasn’t better known by its unofficial name of Internal JTAG or simply IJTAG. (The official title is: “IEEE 1687 Standard for Access and Control of Instrumentation Embedded within a Semiconductor Device.” Now you understand why it goes by IJTAG.) While IJTAG certainly makes use of the IEEE 1149.1 JTAG (boundary scan) standard, the purposes and objectives of IJTAG and JTAG are very different because each one has emerged from a different set of use cases. It’s also noteworthy to point out that the IJTAG standard has enough flexibility built into it so that it can eventually make use of other physical access methods such as I2C or SPI besides JTAG.