1. Current Handling and Saturation Characteristics: The 2STN1360's rated collector current (3A) is double that of the BCP55 (1.5A). More critically, its saturation voltage, Vce(sat), is specified at 500mV @ 3A, which is far superior to the BCP55's rating (the same voltage but only @ 0.5A). Under identical high-current conditions (e.g., >1A), the BCP55 will exhibit significantly higher on-state losses and thermal dissipation, potentially exceeding its safe operating area (SOA) or necessitating a substantial reduction in operating current. 2. DC Current Gain (hFE): The minimum gain of the 2STN1360 (160 @ 1A) is substantially higher than that of the BCP55 (40 @ 150mA). This directly impacts driver circuit design. Substituting with the BCP55 would require a significantly larger base drive current (approximately 4x) to achieve the same output current, which may exceed the drive capability of the preceding stage or increase overall power dissipation. 3. Frequency Response: The 2STN1360 specifies a transition frequency (fT) of 130MHz, a parameter not provided for the BCP55. This indicates the 2STN1360 is explicitly optimized for switching or amplification speed, whereas the BCP55 is likely suited for low-frequency or DC switching applications. Direct substitution in high-frequency circuits could lead to performance degradation or failure. 4. Package Pinout: While physically similar, the BCP55 is explicitly a SOT-223-4 (4-pin) package, typically featuring an additional, separate collector pin for improved thermal and electrical connection. The PCB layout must be carefully verified to ensure pinout compatibility; otherwise, installation errors or incorrect electrical connections may occur.