I googled CLASS 1-99 and this came up all with links to archived info.
Classes of FBI Records in the National Archives
1 - Training Matters
2 - Neutrality Matters
3 - Overthrow or Destruction of the Government
7 - Kidnapping
9 - Extortion
15 - Theft from Interstate Shipment
17 - Fraud Against the Government – Veterans Administration
18 - May Act
19 - Censorship Matters
20 - Federal Grain Standards Act
22 -National Motor Vehicle Traffic Act
23 - Prohibition
25 - Selective Service Act
26 - Interstate Transportation of Stolen Motor Vehicle or Aircraft
27 - Patent Matters
28 - Copyright Matters
29 - Bank Fraud and Embezzlement
31 - White Slave Traffic Act
32 - Identification (Fingerprint Matters)
34 - Violation of Lacy Act
35 - Civil Service
36 - Mail Fraud
37 - False Claim Against the Government
38 - Application for Pardon to Restore Civil Rights
39 - Falsely Claiming Citizenship
40 - Passport and Visa Matters
41 - Explosives
42 - Deserters
43 - Illegal Wearing of Uniform or Decorations
44 - Civil Rights (Racial Violence or Discrimination)
45 - Crimes of the High Seas
46 - Fraud Against the Government - Renegotiation Act - Civil Suits; False Claims - Civil Suits
50 - Involuntary Servitude and Slavery
52 - Government Property - Theft, Robbery, Embezzlement, Illegal Possession, Destruction
54 - Customs Laws and Smuggling
57 - War Labor Dispute Act
61 - Treason
62 - Administrative Inquiries (Miscellaneous Subversive and Non-Subversive)
63 - Miscellaneous Non-Subversive
64 - Foreign Miscellaneous
65 - Espionage
67 - Bureau Applicant Matters
72 - Obstruction of Justice
73 - Application for Pardon after Completion of Sentence
74 - Perjury
77 - Background Investigations (Non-FBI)
81 - Gold Hoarding
84 - Reconstruction Finance Corporation Act
87 - Interstate Transportation of Stolen Property
88 - Unlawful Flight to Avoid Prosecution
89 - Assaulting, Killing or Attempting to Kill a Federal Officer, etc.
91 - Bank Robbery
93 - Ascertaining Financial Ability
94 - Research Matters and General Correspondence
95 - Laboratory Investigative Services
96 - Alien Applicants
97 - Registration Act
98 - Sabotage
99 - Plant Survey
https://www.archives.gov/research/investigations/fbi/classifications