What is the difference between CI( CONTROL INTERVAL)and CA(CONTROL AREA)

The CI is a single unit of transfer from secondary to main memory. Inside it will have freespace to store the records of fixed or variable size.
If u try to insert a new record to a ci which is not having any space to hold at that time the half of the record from this ci will be moved to some free ci. This splitting of ic in case of ci full is called CI splitt.
A collection of ci is called as CA. During the ci splitt if it is unable to find the free ci at that time the entire CA will be splitted into two and half will be moved to some free CA. This is called CA splitt. This is the primary mechanism of the VASM to store all the records inside your ksds in sorted order.