How to create Master-Child Relationship for inventory management?

First step, please create a new SKU value in the ChannelMAX inventory. It should be a non-sellable SKU and labeled as MasterSKU {for example, widget}. This SKU should not be listed on Amazon or any other selling venue. It will simply be used as an umbrella to group the sibling SKUs.

As a next step, we need to create new SKUs to be added under this masterSKU as child SKUs. {such as widget-a, widget-b, widget-c}.

You may refer to the following screenshot to better understand this setup.

In this example, when any one of the 3 SKUs sells, all the sibling SKUs will be deducted from the sold quantity and the quantity for all 3 SKUs will be re-adjusted back to the selling venue. That is, when child SKU 'widget-a' is sold by qty 1, then it will subtract 1 from each of the other two sibling SKUs and ChannelMAX will feed 9 for all 3 SKUs back to the selling venue from the original qty 10.
To update the inventory quantity for all the siblings' SKUs in one go, you need to simply update the inventory of the master SKU and the quantity will be propagated to the children's SKUs.

As per the following screenshot, A01 is the master SKU, hence no MinSellingPrice or ASIN is needed, but Title is needed.
A01-A/A01-B/A01-C are the children of this master SKU A01,
Therefore, MinSellingPrice as well as ASIN are needed, but Title is not needed
MasterSKUControlsInv is Y, meaning that if any of the siblings sells, qty is deducted from all.
MasterSKU is not sellable, hence ASIN is not available.
C-01 and D-01 are independent SKUs and not part of any MasterSKU

File Upload:
You may create a Master-Child relationship via 'File Uploader' too. We need 2 files uploaded as below.

A) Master SKUs file:
SKU (should be populated with MasterSKU value)
SellingVenue
MaxSellingPrice (any value, as this is one of the mandatory columns to upload the file)
Quantity (Same value will be reflected for each child SKU)
Title

B) Child SKUs file:

You may upload a file with different min/max values for each child SKU. The file should contain the following columns.
SKU (child SKU)
Selling Venue
ASIN
MasterSKUControlsInventory (this column should have value Y)
MasterSKU
MaxSellingPrice
MinSellingPrice(optional, needed for repricing only)