Although not mentioned in the CJ1 catalogue, the Interface Control Document for the CJ1 series states:

Section 3.12.2. Requirements of I/O Table Operation

I/O Table shall show the unit profile information even if the CPU's unit profile table includes come CS1 series units.
CJ1 series will be able to connect to the current CS1 base with the CS1 units on the CS1 base. In this case, the CS1's profile codes are stored in the CJ1's CPU unit profile table.

This means that a CJ1 PLC can be connected to a CS1 expansion rack. This vastly extends the capability of the CJ1 series.
The CS1 expansion rack can accommodate compatible C200H I/O and SIOU. However these will be recognised as an empty slot.

The following conditions also apply when a CS1 expansion rack is used:

A CJ1 CPU rack + CS1 I/O base with no empty slot on right side works.

A CJ1 CPU rack + CS1 I/O base with an empty slot on left side causes an I/O setting error.
A CJ1 CPU rack + C200H I/O(Left) and CS1 I/O(Right) at I/O base without empty slot at right side, causes an I/O setting error.
A CJ1 CPU rack + C200H I/O at I/O base without an empty slot at right side results in only the CPU rack working.
A CS1 CPU rack + CJ1 I/O rack causes an IO bus error.