Helpful resources:
MARC21 Concise Format for Bibliographic Data
MARC21 Concise Format for Holdings Data
The MARC21 Formats are also included in Cataloger's Desktop.

Fixed Field
see also ALEPH/NOTIS Leader and 008 Comparison Chart

NOTIS fixed field information is recorded in ALEPH in the MARC21 Leader (LDR) and 008 fields as follows:

Leader: BL, DCF, E/L, RT, STAT, T/C

008 Books: BIOG, CONF, CONT, D/CODE, DT/1, DT/2, FEST, GOVT, ILLUS, INDX, LANG, L/FORM, MOD, PLACE, REPRO, SRC, T/AUD

008 Computer Files: D/CODE, CF/TYP, DT/1, DT/2, GOVT, LANG, MOD, PLACE, SRC, T/AUD

008 Maps: BME, CM/TYP, D/CODE, DT/1, DT/2, GOVT, INDX, LANG, MOD, PLACE, RELIEF, REPRO, SP/FORM, SRC [REPRO is valid for MAP, but not included in NOTIS fixed field]  

008 Mixed Materials: D/CODE, DT/1, DT/2, LANG, MOD, PLACE, REPRO, SRC

008 Music: A/MAT, D/CODE, DT/1, DT/2, F/COMP, LANG, L/TEXT, MOD, PLACE, REPRO, SCORE, SRC, T/AUD

008 Serials: CONF, CONT, D/CODE, DT/1, DT/2, FREQ, GOVT, ISDS, LANG, MED, MOD, OA, PLACE, REG, REPRO, SLE, SRC, S/T

008 Visual Materials: D/CODE, DT/1, DT/2, GOVT, LANG, MOD, PLACE, R/TIME, REPRO, SRC, T/AUD, TECH, T/MAT [A/MAT is obsolete for VIS; REPRO is valid for VIS, but not included in NOTIS fixed field]  

Some NOTIS fixed fields are not recorded in either the Leader or 008: D/S, DT, FMT, R/DT.  In NOTIS, these fixed fields and the Leader appear on the first line of the fixed field area of the bibliographic record.  The 008 information appears on the second and third lines.

See NOTIS/OCLC/MARC21 Bibliographic Record Fixed Field Equivalents for a chart of the corresponding NOTIS, OCLC, and MARC21 codes.

Example:
Green-belt cities by Frederic James Osborn, 1969: NOTIS Record AAC0010, ALEPH Record 9200

UF  FMT B  RT a  BL m  T/C  DT 07/03/82  R/DT 06/01/95  STAT mn  E/L  DCF  D/S D
SRC  PLACE nyu  LANG eng  MOD  T/AUD  REPRO  D/CODE s  DT/1 1969  DT/2
CONT b  ILLUS b  GOVT  BIOG  FEST 0  CONF 0  L/FORM 0  INDX 1

LDR   00704nam^^22002295u^4500
  
Leader number refers to the character position in the LDR field.  Leader numbers begin with 00.  LDR 05 is the sixth character position in the LDR field.

Note: ^ = blank

LDR 05 Record Status (STAT, 2nd position)
n - matches

LDR 06 Type of Record (RT)
a - matches

LDR 07 Bibliographic Level (BL)
m - matches

LDR 08 Type of Control (T/C)
^ - matches

LDR 17 Encoding Level (E/L)
5 - doesn't match; NOTIS is blank

LDR 18 Descriptive Cataloging Form (DCF)
u - doesn't match; NOTIS is blank

UF  FMT B  RT a  BL m  T/C  DT 07/03/82  R/DT 06/01/95  STAT mn  E/L  DCF  D/S D
SRC  PLACE nyu  LANG eng  MOD  T/AUD  REPRO  D/CODE s  DT/1 1969  DT/2
CONT b  ILLUS b  GOVT  BIOG  FEST 0  CONF 0  L/FORM 0  INDX 1

008  691029s1969^^^^nyu^^^b^^^^^b^001^0^eng^^

008 number refers to the character position in the 008 field.  008 numbers begin with 00.  008 06 is the seventh character position in the 008 field.

Note: ^ = blank

008 06 Type of Date/Publication Status (D/CODE)
s - matches

008 07-10 Date 1 (DT/1)
1969 - matches

008 11-14 Date 2 (DT/2)
^^^^ - matches

008 15-17 Country of Publication, etc. (PLACE)
nyu - matches

008 18-21 Illustrations (ILLUS)
^^^b - matches

008 22 Target Audience (T/AUD)
^ - matches

008 23 Form of Item (REPRO)
^ - matches

008 24-27 Nature of Contents (CONT)
^^^b - matches

008 28 Government Publication (GOVT)
^ - matches

008 29 Conference Publication (CONF)
0 - matches

008 30 Festschrift (FEST)
0 - matches

008 31 Index (INDX)
1 - matches

008 33 Literary Form (L/FORM)
0 - matches

008 34 Biography (BIOG)
^ - matches

008 35-37 Language (LANG)
eng - matches

008 38 Modified Record (MOD)
^ - matches

008 39 Cataloging Source (SRC)
^ - matches

Holdings
NOTIS holdings information is recorded in the ALEPH Holdings record and ALEPH Items record.  

Example:
Green-belt cities by Frederic James Osborn, 1969: NOTIS Record AAC0010, ALEPH Record 9200