Siemens SIMATIC S7-300 Series Reference Manual page 29

Connection Resources
Every communication connection requires a communication resource on the
S7 CPU as a management unit for the duration of the communication. Every
S7 CPU has a certain number of connection resources available to it according to
its technical specifications which can be assigned to various communication
services (PD/OP communication, S7 communication or S7 basic communication).
The distribution of connection resources differs between CPUs 312 IFM to 316-2
DP (see the table 3-6) and the CPU 318-2 (see Table 1-8):
PLC S7-300, CPU Specifications CPU 312 IFM to CPU 318-2 DP
A5E00111190-01
CPUs
1-13