The sync engine reads a parsed XER file, which walks P6's WBS tree and task list and turns it into two ordered lists: WBS nodes (parents before children) and tasks. It matches those against what's already in your Smartsheet sheet using a key: the WBS node's synthetic key for header rows, and your Activity ID (ACT#) for task rows.

• First-Time Import walks the WBS tree top-down, inserting header rows and then task rows underneath them, batch by batch, since Smartsheet requires a real row ID for a parent before you can insert children under it. Every task gets its Start written straight from the XER — there's no predecessor-link logic anymore; dates come directly from P6, not from Smartsheet recalculating them.

• Update Sync re-reads the current sheet state, matches existing rows by key, adds anything new, and for everything that already exists: resets formatting to plain black text on every pass (three separate passes, deliberately redundant, because Smartsheet doesn't always confirm a format write took), then re-computes what actually changed since the last run (using a small local cache file, not by re-comparing to the live sheet) and flags only genuinely changed fields red.

• Total Float is written every run but never drives red-flagging — P6 recalculates it relative to the "data date" on almost every export even with zero real schedule edits, so treating it as a change would make everything red, every time.

• Difference Report is a separate read-only pass: it pulls the live sheet, compares Start/Finish per task against the XER, and flags anything that drifted 5+ days or got pushed out by a subtask you added manually in Smartsheet (a row with no Activity ID, indented under a task).

Limitations

• Finish/End can never be written. On a Dependencies-enabled sheet, Smartsheet rejects any write to that column outright — even a blank one — so it's always calculated by Smartsheet from Start + Duration, and the app's row-coloring can't reach that one column.

• A row with manually-added subtasks stops accepting Duration/Start writes. Once you indent something under a task row, Smartsheet treats that row as a parent and calculates its Start/Duration from the children — same lock a WBS header has. The app detects this and simply stops touching those two fields on that row (the Difference Report is how a real divergence there gets surfaced instead).

• No re-parenting. If a task moves to a different WBS section in a newer XER, Update Sync won't move its existing row — it only adds genuinely new rows and updates values in place.

• Conditional Formatting rules (or anyone's manual text-color changes) in Smartsheet take priority over anything the app writes. If a rule or manual formatting is coloring a cell, the app's writes can't override it — that has to be fixed in Smartsheet's own Format menu.

• Holiday/non-working-day list can get truncated. Smartsheet caps how many non-working days a calendar can hold; if your XER's holiday list is longer, the app retries with a smaller list and Duration math is recomputed against whatever actually landed — but this means the calendar in Smartsheet may not be byte-identical to the XER's.

• No live progress visibility into partial failures. Smartsheet's bulk update API is all-or-nothing per batch and doesn't report which individual cell got rejected — so a silent per-cell rejection (like the parent-row lock above) can't be detected or logged, only inferred from known patterns.

• API key isn't saved. You re-enter it each session — nothing is stored in Windows Credential Manager or similar.

• Single-user, single-machine tool. No concurrency handling if two people run a sync against the same sheet at the same time.