After you've proposed or invoiced items, you may only be ready to move forward with purchasing some items now and you'll purchase the rest later. In order to do this you would open the proposal or invoice and in the top right corner click the button with three dots next to "Edit" and then click to "Create Purchase Order(s)". You should then see a window that looks like this:

When asked to confirm the vendor associated to each item, only leave the vendor selected for items which you want a PO created NOW. Any items that you leave or set to "No vendor selected" will not be placed on a PO at this time. Click to continue and only items where you had selected a vendor will be on POs.
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You can always return to the proposal/invoice at a later time and click "Create Purchase Order(s)" again. You'll see this same window, only next time it will show a PO number to the right of any items that are already on POs and will give you the opportunity to select a vendor for the items which you want to create a PO for.