I'll try -
In the first example [basically it's y = x/2] the grade 1 passage indicator is used. If you put a grade 1 word indicator on the y, the space following the y cancels the effect of the grade 1 and you would then need another grade 1 indicator on the opening grouping indicator. It's better for the reader to have less switching (less indicators) and using the passage indicator does that.
In the second and third examples, the numeric indicator following the opening grouping indicator puts the rest of that expression in grade 1 - so the symbol indicator is used before the opening grouping indicator just to let the reader know that is a symbol...and the numeric indicator took care of the rest of those equations.
In the fourth example (the last one on that print page), both of the opening grouping indicators require the grade 1 indicator. Using the grade 1 word indicator creates less switching. It doesn't change the way any of the rest of the equation is read so a grade 1 terminator is not required.
The example on the next page (and still part of 6.4) shows how different types of indicators can be used and still be technically correct. Which way to go on this one would be a transcriber and/or agency decision. Consistency would then be the key there. Choose a method and do all the equations the same way. For me, it would depend on the level of the book I was doing...and whether or not using the contractions could/would cause confusion or provide clarity.