TAM is a documented accredited formal method for conducting process assessments against the TMMi (“Test Maturity Model integration”), the standard reference model for good practices in software quality management and test.
To use the TAM, or have access to it, requires training in its use, which is done through attending Assessor Training. Once training on the TAM has been received (including on the TAM Pack) you are able to license the TAM Pack. (There is training for both Assessors and Lead Assessors.)
The TAM Licence Pack (see here for costs and sample Licence agreement) provides you with a licence to use the TAM and includes supporting artifacts and tools to enable formal and informal assessments to be conducted with confidence. Without an accredited Assessment Method licence pack, you cannot progress to become an Accredited Assessor or be involved in formal assessments.
It is not obligatory to license the TAM Pack; having attended Assessor training, the training received in the use and application of the TAM will be useful in understanding the approach to assessments.