Government document call numbers are different from book call numbers. Government documents use the Superintendent of Documents (SuDoc) classification system. In the SuDoc system, government publications are classified by the government agency that created the document instead of by subject, so all publications from one government department or agency are shelved together.
The first part of a SuDoc number represents the government agency that created the document.
Some examples include A- Agriculture, E- Energy, ED- Educaiton, I- Interior, HS- Homeland Security, X and Y - congress
For complete list click here: https://www.fdlp.gov/catalogingandclassification/cataloging-articles/1791-sudocs-classification?showall=&start=2
Rules for reading SuDoc numbers:
1. Numbers are whole numbers not decimals:
2. Colon before Slash, : before /
3. Dash before Slash, - before /
4. Year, Letter, Number
A 1.35:999 (year, 3-digit)
A 1.35:2004 (year, 4-digit)
A 1.35:R 42/995
A 1.35:R 42/2
5. Nothing before something