The TNM classification system for squamous cell anal cancer uses information about the primary tumor (T), regional lymph nodes (N), and distant metastasis (M) to determine the cancer stage. 

For tumor classification:
- Tx: The primary tumor can't be assessed.
- T0: No primary tumor evidence.
- Tis: High-grade squamous intraepithelial lesion.
- T1: Tumor is 2 cm or smaller.
- T2: Tumor is larger than 2 cm but 5 cm or smaller.
- T3: Tumor is larger than 5 cm.
- T4: Tumor of any size invading adjacent organs (vagina, urethra, bladder).

For lymph node classification:
- Nx: Unable to assess regional lymph nodes.
- N0: No regional lymph node metastasis.
- N1: Metastasis in inguinal, mesorectal, internal iliac, or external iliac lymph nodes.
- N1a: Metastasis in inguinal, mesorectal, or internal iliac lymph nodes.
- N1b: Metastasis in external iliac lymph nodes.
- N1c: Metastasis in external iliac with any N1a nodes.

For distant metastasis:
- M0: No distant metastasis.
- M1: Distant metastasis is present.

Staging is determined as follows:
- Stage 0: Tis with no lymph node involvement (N0) and no distant metastasis (M0).
- Stage I: T1 with no lymph node involvement (N0) and no distant metastasis (M0).
- Stage IIA: T2 with no lymph node involvement (N0) and no distant metastasis (M0).
- Stage IIB: T3 with no lymph node involvement (N0) and no distant metastasis (M0).
- Stage IIIA: T1 or T2 with metastasis in lymph nodes (N1, N1a, N1b, or N1c) and no distant metastasis (M0).
- Stage IIIB: T4 with no lymph node involvement (N0) and no distant metastasis (M0).
- Stage IIIC: T3 or T4 with metastasis in lymph nodes (N1, N1a, N1b, or N1c) and no distant metastasis (M0).
- Stage IV: Any primary tumor size (Tis, T1, T2, T3, or T4) with any lymph node involvement (N0, N1, N1a, N1b, or N1c) and distant metastasis (M1).

Each stage reflects the progression and spread of cancer, aiding in determining the treatment approach and prognosis.

The calculator in question is the TNM Classification for Squamous Cell Anal Cancer. TNM stands for Tumor, Node, Metastasis, and it is a globally recognized system used for classifying the extent of spread of cancer. This particular classification is specifically applicable to patients diagnosed with squamous cell anal cancer.

The clinical utility of the TNM Classification for Squamous Cell Anal Cancer lies in its ability to guide treatment decisions and provide prognostic information. It helps clinicians to stage the cancer based on the size and extent of the tumor (T), whether the cancer has spread to the lymph nodes (N), and whether it has metastasized, or spread to other parts of the body (M). 

There are no specific exclusion criteria for its use, but it is important to note that the TNM classification is less useful in early-stage cancers where the extent of disease is minimal. Additionally, it may not be as accurate in predicting prognosis for patients with rare types of anal cancer other than squamous cell carcinoma.

TNM classification for squamous cell anal cancer