Personal Computers especially laptop computers have lithium ion batteries inside which potentially fall into the category of dangerous goods.

Unless you obtain an SDS (Safety Data Sheet) stating that the concerned product, especially the batteries, are not hazardous when shipped by air, lithium ion batteries must be removed.

One difficult and unique thing about shipping Apple products including iPhone and iPad is that Apple does not issue SDS for their products.  Since airlines ask for SDS, shipping these would not be possible. (Apple says that their products are all safe and shipped worldwide, however, when their products are shipped individually, it is a different story and airlines ask

for the same documents as they would do for any other products.

In the case of iMac, the main body does not have any batteries so it is okay but you need to be careful about magic keyboard and mouse.  These contain lithium ion batteries so it is important that you remove batteries from them.

When shipping an iMac by air, it would be okay if you provide reference numbers of the products and indicate them on the packing list.

Apple products normally have various numbers.

One is like Model Name like iMac (Retina 5K, 27-inch, 2017)

Another one is like product ID which is something like iMac (Retina 5K, 27-inch, 2017)→ iMac18,3.

Also, a model number which is  like MNDY2J/AJ. This is the most important one. This model number is commonly used worldwide and tells you what model it is. Normally the first letter begins with M. If it is a product repaired by Apple, then it begins with N. The next part, which is like, NDY2 is the unique number for the model and is common worldwide as long as the model is the same. Then there is the alphabet that tells where it was made.

The example has the alphabet J and it means it was made in Japan.

So, when shipping an iMac it is important to provide all such details on the packing list.