Import the lead time from your ERP system at the Item - Location level if available. Alternatively, if you are providing the csv files, use the lead time provided in the stock.csv file.
This would be known as the ‘Planning Lead time’ and would be set up in the Configuration settings as ‘Import Supplier Lead Times’ enabled and ‘Source of item Lead Times' set to ‘Imported by Item’.

Import the lead time from your ERP at the Supplier level if available. Alternatively, if you are providing the csv files, it would be the lead time that is provided in the supplier.csv file. This lead time will then be applied to all the items that have that supplier or vendor assigned to them in the daily update.
This would also be known as the ‘Planning Lead time’ and would be set up in the Configuration settings as ‘Import Supplier Lead Times’ enabled and ‘Source of item Lead Times' set to ‘Set by Supplier’.

Set the Lead time by Supplier in the App. This is not really optimal unless you have a handful of suppliers that can be manually maintained in the App. It also means that that data resides in the App and if for any reason the App is no longer your choice of inventory optimization software, then that data is no longer available to you.
This Lead time then also gets assigned to all items linked to that supplier in the daily update and is known as the ‘Planning Lead time’ and would be set up in the Configuration settings as ‘Import Supplier Lead Times’ disabled and ‘Source of item Lead Times' set to ‘Set by Supplier’. This way the system knows that the setting is in the App and not imported.

Calculate the Lead time using the historical data.
This is slightly more complicated than it sounds. Theoretically - it is the time lapsed between the order date and the receipt date on all the orders that we have in the database, after excluding the exception covered in the article Correcting imported data - purchase order history.

This is set up in the Configuration settings as ‘Import Supplier Lead Times’ disabled and ‘Source of item Lead Times' set to ‘Measured by historical deliveries'. This way the system knows that the setting is in the App and not imported.
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The App only considers imported historical purchase orders against the supplier currently identified as the preferred or default supplier that you wish to plan against.

The App then ‘excludes’ blanket orders against that supplier. A blanket order is identified when an item has 2 or more records where the order date, the location and the supplier are the same. This means means that that item was captured more than once on the same order, potentially just with multiple quantities and different delivery dates. E.g. I require 10,000 units, but require 1,000 per month, over 10 months - often referred to as a call off order.
This could skew the calculations considerably, as the lead times against this order could vary substantially, with multiple receipt dates referring to the same order date. As a result, the App takes only the first delivery into account and not the rest.

The App then has a look at the range of the deliveries, and exclude outliers using standard deviations, to derive the average of all the deliveries over time.

It is preferable to use the data for that item, in that location from that supplier to do the calculations, but it is possible that once the App has run through all the exclusions above, the data set is too small.

If there is not sufficient data i.e. more than 2 records, the combination of the supplier into that location is used.

If there are still not enough records for that, the average of that supplier across all locations is used.