SEBI - DIP guidelines IPO by Book Building

#1
As per SEBI DIP guidelines on bookbuilding
There are two options available to the issuer Company

(1) 75% Book Building process and
(2) 100% book building process - Which provides for
(a) 100% of the net offer to the public through BB or
(b) 75% of the net offer to the public through BB and 25% @ the price discovered through BB.

What is the difference between (1) and (2)(b)? - Bold and Underlined

Besides in the 75% BB process there are these clauses
Clause 11.2 (ii) (a) & (b)

The BB facility is available as an alternative to and to the extent os the % of the issue reserved for firm allotment as per these guidelines

The issuer company will have the option of either reserving the securities for firm allotment or issuing the securities through BB

What is the meaning of these two clauses? and what is the % of firm allotment that can be made?

will be grateful if some one helps me... Thanks !!!