Acknowledgements to Andy for doing the "heavy lifting" here. I'm relying on fthe definition he's posted. And he's absolutely right about sustitutos being called alternates in this context.

This distinction seems to exist in Scandinavian countries, where the terms "personal alternate" and "non-personal alternate" can be found, as for example in this document from Norway:

"The employee representatives on the Board of Directors have the following alternate members: Jon E. Kristiansen (personal alternate member for Mr. Nilsen), Silje Johnsen (personal alternate member for Ms. Kristiansen), [...] In addition, Lena B. Lorentzon and Gaute Ytterstad are non-personal alternate employee representatives to the Board of Directors."
http://www.recgroup.com/PageFiles/3126/Laantakerbeskrivelse ...

I think this is a viable option, but of course it's a translated document, and it would be nice to find an original EN use of the term. I can't find one, though I wouldn't rule it out.

The term I've found most often is "general alternate". Here are a couple of examples:

"Vacancies. Any vacancy or vacancies except the president's, in the Board of Trustees, whether caused by death, resignation or removal will be filled with the first, second or third general alternates in that order [...]"
http://www.jerseywatch.com/uploads/team_documents/Bylaws_-_M...

"F. Executive Committee
[...]
2. Three general alternates to the executive committee shall be elected annually by the board of trustees."
http://www.dordt.edu/publications/faculty_handbook/faculty_h...

"Members chosen by the Students’ Representative Council
[...]
Ten students (of which three must be graduate students), and up to ten specific or general alternates"
http://www.rcips.uct.ac.za/sites/default/files/image_tool/im...

However, in your context, I think "the alternates will be general" would sound odd and might not be understood. I would suggest calling them "non-specific" in this case. As my last reference (from South Africa) shows, personal alternates are sometimes called specific alternates. Well, there can also be non-specific ones.

I have a source for this, but only one: the rules of the Democratic Party of the State of Maine (1984):

"Each municipal caucus shall elect a specific alternate of the same sex for each delegate [...]
3. Should a delegate fail to attend the State Convention, his or her specific alternate shall act as a delegate. Should both a delegate and his or her specific alternate fail to attend the State Convention, and available alternate of the same preference shall be selected to act as delegate. If no alternate of the same preference from the same municipality is present, any available alternate from the municipality shall be chosen. Such choices of non-specific alternates shall be made in the order in which the altenates’ names appear on that municipality’s official list."
http://maineanencyclopedia.com/democratic-party-rules-1984/

There may be another term I haven't thought of, but it doesn't seem to be a distinction commonly made in English.