# PIP-0004 · Composable liquidity, still reserved

Keep the settlement `route` field empty until a later PIP specifies who may fill it.

## Why this exists

`route` is on every settlement receipt. In this version the value is `null`. [PIP-0005](/pips/0005) defines the only shape the field may take.

This PIP records the refusal to fill `route` in v0.1, and the conditions under which a later PIP may.

## Rule, for now

Implementations MUST accept `route` and MUST leave it empty. A non-empty `route` in v0.1 is a non-conformity. PATH does not run a rail, and a network that puts its own corridor into `route` is inventing a PATH Liquidity that was not specified.

## When a later PIP may fill it

Only when all of these are true:

1. The field has a single canonical meaning across networks.
2. A receipt with a route and a receipt without one remain verifiable as the same kind of object.
3. Filling it does not require PATH to admit members or to hold balances.

[PIP-0005](/pips/0005) writes the canonical shape (`legs`, `providers`, `eta_seconds`). Condition 1 is met. Conditions 2 and 3 are unchanged: a receipt with a route and a receipt without one stay the same object, and filling it still must not require PATH to admit members or hold balances.

Until a later PIP names who may write a non-null `route`, implementations MUST emit `null`. PATH LIQUIDITY stays reserved.