The construction suposto que is at first glance similar to the English construction in sentences like "He is supposed to....".
suposto que supposed
É suposto que aceitemos essa desculpa também este mês?
It is supposed, that we will except the excuse as well this month?
In both cases the past participle suposto / supposed has its own subject, it. In Portuguese, as in any other roman language there is no need to mention it, it is implicetly expressed by the verb. The question is, who is it? In English we have to distinguish between two different situations.
1) He is supposed to do it.
2) It is supposed that he will do it.
1) is not possible in roman languages and presumably in no language. Suppose requires a direct object. The direct object can become the subject of past participle, but not the nominative. (At least in any other language.)
2) is possible in any other roman language and presumably in a lot of other languages. The question is, who or what is it? In a sentence like "It is supposed that he will do it" the direct object is "that he will do it". (One suppose that he will do it.) The whole subordinate clause "that he will do it" can become the subject of the past participle. "It" is therefore a personal pronoun and refers to this subordinate clause. This is actually a complicated structure in theory, although nobody has any problem understanding it in practice. We even understand such a complicated structure in a foreign language without even analysing it. One can wonder how this works and a possibel answer is, that the way the human brain wants to verbalise the world is the same everywhere, that it is "genetically" determined.
suposto que supposed
Não é suposto que o faça.
It is not supposed that he will do it.
É suposto que faça um grande discurso que fale das coisas boas do casamento.
It is supposed that he will held a great speech talking abut the beautiful things of marriage.
Foi suposto que o objetivo do sistema seria o aproveitamento da energia natural.
It was supposed that the purpose of the plant was the supply with renewable energy.