All the id spaces xmls will get combined on server startup.So if any item-descriptor need any id space reference, they will search it in ipspace.xml.
At Item-descriptor level there is a property 'id-space-name="xyz".If id-space-name is encountered in repository xml, ATG will serach xyz in idspace.xml.
I think even if we don't specify id-space-name, It will search based on item-desc name.
Seed is the first id.
Batch is slot of id reserved.
Refer link for id space IdGenerators and IdSpaces
i.e say u have seed 1 and batch 100.
Id generator will start generating ids from 1. It generates all ids till 100 or till server restart. Once ids reached to 100 or server is restarted, It will start generating ids from 101.
<id-space prefix="abc" name="xyz" seed="1" batch-size="40000" />
Since u have given prefix in id-space, So Id generator is generating ids with prefix abc.
5545 is in first slot of ids 1-40000.
For better understanding refer SQLIdGenerator
Now if u restart server, IdGenerator will generate ids from slot 40001-80000.