Add requirements for probability sampler

Signed-off-by: Bogdan Drutu <bogdandrutu@gmail.com>

specification/trace/sdk.md

@@ -125,6 +125,13 @@ TODO: Add details about how the probability sampler is implemented as a function
 of the `TraceID`.
 TODO: Split out the parent handling.
 
+##### Requirements for `Probability` sampler algorithm
+
+* Ensures that sampling algorithm is deterministic. A trace identified by a given `TraceId` is sampled or not independent of language, time, etc. To achieve this implementations MUST use the random part of the `TraceId` when applying probability sampling.
+  * By ensuring this, even if the user configures the `Probability` sampler to run for all the `Spans` the same number of traces will be sampled.
+* Systems should be able to implement inflationary probability rate. To achieve this a `Probability` sampler with a given sampling rate should sample all traces that any `Probability` sampler with a lower sampling rate would sample.
+  * This is important when a backend System may want to run with a higher sampling rate than the frontend System, this way all frontend traces will still be sampled and extra traces will be sampled on the backend only.
+
 #### ParentOrElse
 
 * This is a composite sampler. `ParentOrElse(delegateSampler)` either respects the parent span's sampling decision or delegates to  `delegateSampler` for root spans.