Siemens S7-1500 Function Manual page 30

Bit memories, timers and counters
You can define the number of retentive bit memories, timers and counters in STEP 7 in the
PLC tag table using the "Retain" button.
Figure 2-7
Reference
You can find additional information on setting the retentivity in the STEP 7 online help.
Structure and Use of the CPU Memory
Function Manual, 10/2018, A5E03461664-AC
Definition of the number of retentive bit memories, timers and counters (beginning at 0,
continuing without gaps) using the "Retain" button
Memory areas and retentive memory
2.3 Retentive memory areas
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