| Abbreviation | Definition |
|---|---|
| RTAPS | Rural Time and Attendance Processing System |
| RU | Russian Federation |
| RW | Rwanda |
| S/SIS | Sales/Service Information System |
| SA | Saudi Arabia |
| SAE | Self-Addressed Envelope |
| SAM | Space Available Mail |
| SASWYB | Semi-Automated Scan Where You Band |
| SB | Solomon Islands |
| SBM | Stamps By Mail |
| SC | Seychelles |
| SCF | Sectional Center Facility |
| SD | Side |
| SD | Special Delivery |
| SD | Sudan |
| SDO | Stamp Distribution Office |
| SG | Singapore |
| SH | St. Helena |
| SHIPT | Shipment |
| SHR | Shore |