Certain data item labels are reserved and refer to data items that may be needed by NONMEM or PREDPP:

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   ID L1 L2 DV MDV
   RAW_ MRG_ RPT_
   TIME DATE DAT1 DAT2 DAT3 DROP SKIP EVID AMT RATE SS II ADDL
   CMT PCMT CALL CONT

;  PREDPP data items
   TIME EVID AMT RATE SS II ADDL CMT PCMT CALL CONT
   XVID1 XVID2 XVID3 XVID4 XVID5 (NM72)

The RAW_ data item identifies template records for which NONMEM computes and displays raw-data averages. With this feature, the TEMPLT variable may be used in abbreviated code, and the $OMIT record may be used.

The MRG_ data item identifies records for which NONMEM computes and displays marginal quantities (expectations).

The RPT_ data item identifies NONMEM's repeat data item. It is used to mark a data record as a repetition base. (Another way of doing this is via global "Repetition Variables" RPTI, RPTO, RPTON, PRDFL in abbreviated code.)

The REPL_ data item identifies NONMEM's replication data item. NONMEM replicates subjects from the template data set at the start of the problem. REPL_ may be used with the $DATA ... REPL option.

By using ID, L1, L2, DV, MDV, RAW_, MRG_, RPT_, or REPL_ as B, the user defines the NONMEM data item type whose name corresponds to the label. The NONMEM data item type whose name corresponds to ID is the same NONMEM data item type whose name corresponds to L1, but L1 has another special meaning. Use of L1 not only defines the ID data item, it also supresses automatic generated ID data items (see section II.C.4).

By using TIME as B, the user defines a time data item type; time data items can be recognized as clock times and translated to relative times (see section II.C.2 and the discussion below).

By using DATE, DAT1, DAT2, or DAT3 as B, the user defines a date data item type (see section II.C.2 and the discussion below). Usually, date data items should not appear in the NONMEM data set (see below).

By using DROP as B, the user defines a data item type which will not appear in the NONMEM data set, e.g. DATE=DROP. This is the one label that can be used more than once in the $INPUT record. Ignoring items with this label, the total number of items in a NONMEM data record cannot exceed a certain limit, and, if the data set is single-subject, this limit includes generated ID data items (which actually do not appear in the NONMEM data set).

Starting with NONMEM 7.2, this limit is given by PD in resource/SIZES.f90. The default value is 50. PD may be changed using the $SIZES record. PD also specifies the maximum number of data items in the NONMEM data set. With previous versions, the maximum number of items in both NONMEM and NONMEM data sets was 20.

Certain data item labels are semi-reserved, in that they have reserved meanings if used in $INPUT, but can also be user-defined in abbreviated code, in which case they have no reserved meaning, e.g.

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   XVID1 XVID2 XVID3 XVID4 XVID5
   REPL_

Here XVID's are extra EVIDs.

Certain labels should not appear in the $INPUT record:

• ETA1 ETA2 ETA3 ETA4 ETA5 ETA6 ETA7 ETA8 ETA9
• Labels for basic PK parameters (e.g., CL, V, K, KA)
• Labels for additional PK parameters (e.g., S1, F0, R1, F1)
• Arguments of subroutines internal to NONMEM/PREDPP MU variables (e.g., MU_ MU_i )

If the user prefers to label a NONMEM data item type or a time data item type with a label (A) other than the reserved one (B), he may use the item A=B. In this case A is called a synonym for B. Alternatively, he may use the item B=A, i.e. the order of the labels A and B are reversed in the item so that the reserved label comes first. Either of the labels A and B may be used in subsequent control records of the problem specification. However, only the synonym is used as a label in NONMEM output. Both A and B can be reserved labels. Thus one type of data item can serve simultaneously as another type, e.g. ID=TIME. Another example of this is DATE=DROP.

The items DROP, A=DROP, or DROP=A are all equivalent; the label SKIP acts just as does the label DROP; the label DROP may not be used when the $DATA record contains a format specification.

If DROP (or SKIP) is used as a data item label or synonym, the data item type will not appear in the NONMEM data set. DROP (or SKIP) may be used with more than one item.

If the label DATE (or DAT1, DAT2, or DAT3) is used, DATE=DROP causes the data item to not appear in the NONMEM data set. However, NMTRAN will adjust the TIME data item to reflect the date.

Time data items are translated from clock times to relative times when at least one time data item contains a colon (:). Time data items are also translated from clock times to relative times when the reserved label DATE, DAT1, DAT2, or DAT3 is used. In this case the order of the fields (day, month, year) must be the same across all the date data items. This order corresponds to which label is used:

If only one field is used, it is assumed to be the day field. If two fields are used, they are assumed to be month and day fields. When two or more fields are used, the user should be careful to use, for example, DATE=DROP; otherwise, use of the nonumeric separator will raise an error during the NONMEM run.