The conjunction salvo se formulates, simmilar to a não ser que, the condition negative. These sentences have two completely different meanings.
He talks a lot, except if he is tired. .
He talks a lot, if he is tired.
Salvo se is transparent, the two components suggest the actual meaning. Salvo means except and se if. All togehter we get except if and that is the actual meaningg. Since the conjunction se requires the futuro do conjuntivo if the event is imagined as happening in the future, the compound conjunction salvo se requires the futuro do conjuntivo as well. Given that in the case of regular verbs there is no difference in the form between the futuro do conjuntivo and the infinitivo pessoal it can be difficult to figure out whether a futuro do conjuntivo is used or an infinitivo pessoal. (A distinction we have to make in case of irregular verbs. In these case we have to know whether we need a futuro do conjuntivo or an infinitivo pessoal. For a detailed discussion see 16.1. formation of the infinitivo pessoal.) The easiest rule is this. Whenever we can substitute the verbal form by a noun, it is an infinitivo pessoal. The same syntactical function as the infinitivo has the English gerund.
salvo se except if
Não empreendemos nenhum projecto novo, salvo se este for* efectivamente necessário.
We will not start new project, except if it is absolutely necessary.
Salvo se for estritamente necessário, os cookies não contêm qualquer informação pessoal.
Except if it is absolutely necessary, the cookies doesn't contain any personal information.
* for is futuro do conjuntivo, not infinitivo pessoal. Infinitivo pessoal 3rd person singular would be ser. The difference exists only in the case of irregular verbs. It would be possible as well to construct with an infinitivo pessoal, however in this case we have to construct with a preposition: "Não empreendemos nenhum projecto novo, salvo este ser efectivamente necessário". In this case the 1st and 3rd person singular couldn't be distinguished from the infinitive. In the 1st and 3rd person singular there is therefore no need to know whether the infinitivo pessoal or the infinitivo is to be used. Concerning the other persons it depends whether the subject of the infinitivo (pessoal) differs from the subject of the main clause, see chapter 16.
The picture is not always the same in all roman languages. In Italian for instance salve se requires the indicativo (decide).
Il Consiglio si riunisce due volte all'anno, salvo se decide diversamente.
The council meets once a year, except if decided otherwise.