An MIPU can go into all 8 slots in the base cabinet. Per the install guide an MIPU should only be in slots 1-6 in the expansion (assuming an MIPU 16), and you are also limited to 5 MIPU card per expansion cabinet.

Slots 7-10 in the expansion cabinets only support 8 port cards, such as a BDKU, RCOU, RSTU, or BIOU. If someone put an MIPU in slot 7 of the expansion then this is technically incorrect. It will probably work if slot 8 is open, but it is usually best to follow Toshiba's guidelines if you want to avoid problems.

Max MIPU cards per a 7 cabinet system is 36 MIPU cards, or until you reach 560 maximum extensions.