What do all of the HL7 segment structure table headers mean?

Question

For example, the OBR segment reference table from here has the following headers:

• SEQ
• LENGTH
• DT
• OPT
• RPT/#
• TBL#
• NAME

Here are my observations:

• SEQ seems fairly self-explanatory: It's the field number in the segment. Things like OBR.1, OBR.2, OBR.3, etc.
• LENGTH looks fairly straightforward too: It's the length of the data. But some of them are 0?
• DT looks like data type. Things like SI, EI, ST, XCN, ID, NDL, etc.
• NAME is also self-explanatory: It's the full name of the field.

Here are the main things I don't know about:

• What does OPT mean? In this column, I've seen things like: O, R, RE, C, B, X, W
• What does RPT/# mean? In this column, I've seen things like: *, 1, 2
• What does TBL# mean? In this column, I've seen things like: 0065, 0074, 9999

Answer 1

• LENGTH looks fairly straightforward too: It's the length of the data. But some of them are 0?

Lengths are authoritative or arbitrary. Some lengths are exact lengths. Some are maximum lengths. That is why some of them are zero. Few other guides simply put it empty. Refer to this.

• What does OPT mean? In this column, I've seen things like: O, R, RE, C, B, X, W

OPT denotes optionality.

R = Required  
RE = Required but may be empty  
O = Optional  
C = Conditional  
X = Not Supported  
B = Backward Compatibility  
W = Withdrawn

Refer to this for more details.

• What does RPT/# mean? In this column, I've seen things like: *, 1, 2

This denotes how many times the field value can be repeated. In other words, how many values a field can hold.

1 = Only one value  
2 = Maximum two values  
* = Not Defined/Multiple

• What does TBL# mean? In this column, I've seen things like: 0065, 0074, 9999

Those are table numbers where more information about the field should be referred. So, in your example, for OBR.11 field, linked table is 0065 which lists the possible values for that field:

A = Add ordered tests to the existing specimen  
G = Generated order; reflex order  
L = Lab to obtain specimen from patient  
O = Specimen obtained by service other than Lab  
P = Pending specimen; Order sent prior to delivery  
R = Revised order  
S = Schedule the tests specified below