The aim is that the money should be used to support eligible pupils, helping them achieve their full potential. Obviously all children are different, and so there are many ways the money could be used. A good school will be engaged with the parents in the process of finding ways to support the child, and how the PPP might be used. It is the schools responsibility to spend the money for what it has been intended to do. I do appreciate that some schools are terrible at this, and I advise making a complaint about it.
The money is not ringfenced and it doesn't have to be all used on an individual child, however the school are accountable to OFSTED. In some cases, the money might be totally spend on things that are very individual for a particular child. Other times it might be considered a good idea to, for example, use money to send a few teachers on a training course in how trauma presents in the classroom/attachment/etc, if that would help the child indirectly. Alternatively, sometimes a whole-class use of some money might be appropriate, as long as that intervention will definitely be benefitting the child the money is meant to support where they have a need (and OFSTED will want the school to show them that was the case). What the school must not do, is allow the money to be used for general stuff or just be swallowed up into the main 'pot', which will not help support the child. The spending must be targeted. Again, I really think a formal complaint should be made if a school are deliberately not spending any of the money on the individual child.
I also think that parents should try and schedule a meeting with school early on, preferably before the school actually get the money. I met with my DS school, who are admittedly very good at listening to me, and we basically sat around the table and identified where we thought DS's main challenges lay, and then talked about what might be useful for him.
So, is your child behind in certain subject areas much more than others and would they benefit from extra support with those subjects? Or, like my DS, do they struggle more with their behaviour, managing their emotions or social skills? (That's where DS school are targetting the intervention. We're all very much on the same page that this is his most pressing difficulty in school). Money can also be spent on things like providing teacher training on a specific issue eg. managing attachment issues/disorder in the classroom. Or on providing equipment access, if your child needs that. The school can pay a third party to provide something to a child, which cant be provided by anyone at school. The support doesn't even have to be provided within school hours if there's an intervention that would be better provided 'out of hours'. Home tuition, extra-curricularum trips etc. It totally depends on what your child needs to help them reach their potential in school.
The Sutton Trust's pupil premium toolkit can be used by the school. It shows schools which kinds of interventions are supported by evidence etc
A poster called tethersend knows a huge amount about education and how this all works, so hopefully she will be able to help