File number vs client number

A client ID number is a reference that Immigration Canada will assign to the sponsors application, by using this reference on their website, the sponsor will be able to verify the status of the sponsorship & also the progress of your immigration application once it reaches the visa office.

It is the reference that the sponsor or the applicant must use, when communicating with immigration Canada about the sponsorship, & it is also attached to any dependent secondary processing such as "immigration application forwarded to a visa office"

All application that are processed inside Canada ex: (sponsorship part, student authorization, work permit) that uses that OLD BROWN document contains a client-ID. A client ID is also the reference to provide when paying the fees using the Pink IN-CANADA immigration receipt, if a client ID has been assigned already.
Remark: the sponsor can also use any of the receipt numbers used to pay the fees of the application, to verify the overall status Sponsorship+immigration using the E-Cas from immigration Canada. But remember that you have to enter the sponsor's information to verify your identity, since the sponsor is the CLient, ... the sponsor is also the persone hat must pay the Permanent residency fee.

An immigration file number is the reference given by the visa office processing the immigration part of your application, & it usually starts with B followed by 9 numbers. it is only for communicating with immigration Canada with the person who is being sponsored.

When using this in the website to check the status of the application, you must provide the complementary details of the person who is being sponsored (ex: date of birth, names, country of birth) .. & this will only show the 2nd immigration part that is processed at the visa office responsible of your country or Application.

Good luck.