Alternative provision is any education outside of a school - it can mean a PRU, 1:1 tuition or projects in the private sector. They will not be regulated in the same way as schools are and may not offer the national curriculum.
It's worth looking at some of the official guidance on these.
PRUs generally take children who have been excluded so it may not be suitable for those with SEN.
This is what it says in the Education Act regarding alternative provision:
"(1) Where a local education authority are satisfied that it would be inappropriate for?
(a) the special educational provision which a learning difficulty of a child in their area calls for, or
(b) any part of any such provision,
to be made in a school, they may arrange for the provision (or, as the case may be, for that part of it) to be made otherwise than in a school.
(2) Before making an arrangement under this section, a local education authority shall consult the child?s parent. "
i.e. it has to be the last resort after all possible school placements have been considered and you should be consulted as a parent.
If you feel that there is a school that is more likely to meet your child's needs and that school has offered her a place, you therefore have a strong argument against the alternative provision. LAs sometimes name alternative provision because they want to avoid paying for independent special schools.