51Did (51Degrees Identifier) and PMP (Privacy Marketing Preference) are derived signals downstream systems can act on without seeing the raw inputs.

51Did - signed identifier (a base64 OWID envelope) carrying a probabilistic value, derived from three inputs: the Device ID (a Hardware-Platform-Browser-IsCrawler tuple produced by Device Detection), the client IP, and the usage purpose (non-marketing, standard, or personalized) declared per request. See 51Did (51Degrees Identifier) for the identifier-versus-value distinction.
PMP - embeddable widget that captures a marketing preference (standard / personalized) suitable as id.usage input to 51Did.

Flow

User → PMP → preference → id.usage → 51Did → downstream

Use one without the other when

PMP alone - you need to collect a user marketing preference and act on it directly (e.g. gate ad personalization, drive a paywall), without feeding it into 51Did.
51Did alone - id.usage=non-marketing (e.g. fraud or suspicious activity); the preference is set by the integrator, not the user.

See 51Did (51Degrees Identifier) and PMP (Preference Management Platform). 51Did depends on Device Detection Overview.