I'm assuming that this is a gemeinschaftliches Testament, probably between spouses.

I would translate this sentence as follows:

However, the survivor [surviving spouse/party] shall be entitled to unilaterally, by way of a last will and testament, dispose of an aggregate value/amount of 20% of his/her total assets - net assets - remaining, after deduction of the bequest in accordance with Clause 2a above, as at the time of his/her death [decease], in the form of monetary and/or material bequests as testamentary gifts and legacies to any given person in any given share.

The usage of dashes is very confusing - I would probably do a better job, if I saw the original document. Anyway, the gist is that 20% of whatever is left after the deduction of the first set of bequests can be left by the survivor to whomever they wish, again by bequests in the form of money (which is usually called a legacy) or in the form of specific items of property (I wouldn't worry about the distinction between bequest and legacy, they are often used as synonyms). Phew ...