|As razor points out, both examples are functionally identical.|
I think what you may be missing is the idea of "nesting" itself, basically all it means in this context is an "if statement being executed by another if statement".
Formatting is irrelevant, it's all about the logic of the statements. Nesting if statements, with or with out else clauses, is used to ensure multiple conditions are met.
That said formatting plays an important role in readability and each format has it's strengths.
In my opinion the single line if-else gets confusing when nested, using full code blocked style is good for nest if without else and the inline if-on-else is good for nested if else:
if 1==1 (do command) else do command
if 1==1 (
if 2==2 (
if 3==3 (
if 1==1 (
) else if 2==2 (
) else if 3==3 (
Of course it all depends on how you are using it, a big factor is what you have to execute, multiple commands sit well in code block while with a single command they can be unnecessary. The styles above are just preference, to each his own.
As for the mention of compatibility with 9x batch script, I very much doubt it. On those systems there is no else clause or code blocks and the statement must be contained in one line. The only way 9x if statements can be nested is like so:
if 1==1 if 2==2 if 3==3 echo Yay!
Line length limits catch up pretty quickly on those systems though....
Batch Variable how to