This section outlines how the terms "comprising" and "consisting of" are to be interpreted when construing a claim.
A claim directed to an apparatus/method/product "comprising" certain features is interpreted as meaning that it includes those features, but that it does not exclude the presence of other features, as long as they do not render the claim unworkable.
On the other hand, if the wording "consist of" is used, then no further features are present in the apparatus/method/product apart from the ones following said wording. In particular, if a claim for a chemical compound refers to it as "consisting of components A, B and C" by their proportions expressed in percentages, the presence of any additional component is excluded and therefore the percentages must add up to 100% (see T 711/90).
In the case of chemical compounds or compositions, the use of "consisting essentially of" or "comprising substantially" means that specific further components can be present, namely those not materially affecting the essential characteristics of the compound or composition. For any other apparatus/method/product these terms have the same meaning as "comprising".
Regarding Art. 123(2), "comprising" does not provide per se an implicit basis for either "consisting of" or "consisting essentially of" (T 759/10).