In simple language Assignment means Transfer. Trademark Assignment means selling of trademark.

As we know that trademark is an intellectual property of the owner, as other property can be sold for a price, the same way a trademark can be sold for a price.

The selling of trademark does not mean selling of the company. A company may have many brands under its ownership.
For example: Unilever owns more than 400 brands, hence it may choose to sell few brands out of its portfolio.
1) Assignment Deed: As any property, may be a car or house is sold by an agreement between the buyer and the seller, the same way trademark is transferred by an Assignment deed between the buyer and the seller.
2) Registration of Assignment Deed with Trademark registry so as to record the buyer name as owner of the trademark in the Trademark Department’s records.

1) Selling Price or Consideration for transfer of trademark is very important clause based on which stamp duty of the Assignment deed is calculated
2) Geographical area within which assignee or purchaser can use the brand
3) Information if the transfer is with Goodwill or without Goodwill