rev id and read_rev_id() function
You haven't provided enough information for anyone to easily help you. Assuming that you took your code from GitHub myAHRS_plus_i2c.ino you can look further down the file to find the definition of read_rev_id(). It simply reads the revision number from the AHRS chip and displays it on the console. Since this is all that it does you could leave out the call to read_rev_id(). But if you are going to use it then be aware that it contains an obvious bug. The second read() should be to I2C_SLAVE_REG_REV_ID_MINOR.

//READ REVISION ID
int read_rev_id()
{
    uint8_t id_1 = 0;
    uint8_t id_2 = 0;
    read(I2C_SLAVE_REG_REV_ID_MAJOR, &amp;id_1, 1);
    read(I2C_SLAVE_REG_REV_ID_MAJOR, &amp;id_2, 1);
    Serial.print("READ REVISION ID= "); Serial.print(id_1); Serial.println(id_2);
}