What means “fallbacks” in LN invoices?

Question

Just saw a "fallback" property in LN invoice. Never seen before. Can you please describe what that means and how I can make an invoice with same property?

I am using c-lightning. This is what i had decoding invoice:

{
   "currency" : "bc",
   "created_at" : 1564513459,
   "expiry" : 3600,
   "payee" : "03efccf2c383d7bf340da9a3f02e2c23104a0e4fe8ac1a880c8e2dc92fbdacd9df",
   "msatoshi" : 1000000000,
   "amount_msat" : "1000000000msat",
   "description" : "satoshis-tennis-1564513459.3110697",
   "min_final_cltv_expiry" : 10,
   "fallbacks" : [
      {
         "type" : "P2SH",
         "addr" : "3MXqbMwf457U4Jaw35WnJdnL99mq7Q8oQQ",
         "hex" : "a914d9a625fb64a9faa4600548ae97e8efda9294dd0a87"
      }
   ],
   "routes" : [
      [
         {
            "pubkey" : "0276e09a267592e7451a939c932cf685f0754de382a3ca85d2fb3a864d4c365ad5",
            "short_channel_id" : "557233x2338x0",
            "fee_base_msat" : 1000,
            "fee_proportional_millionths" : 9,
            "cltv_expiry_delta" : 14
         }
      ]
   ],
   "payment_hash" : "de210472f7e9513358a22db99ee5b987e9fe96d523d0d833fd900837aa71199a",
   "signature" : "3045022100c79fc20087480d302a68c3b92b34de935956202c0499cf8acc182837c871923202200879fd3979aa688a46703c31f7e56ea5e7917f4820da42177d31fc2a9290c91a"
}

I am in particular interested in the following part:

    "fallbacks" : [ { "type" : "P2SH", "addr" : "3MXqbMwf457U4Jaw35WnJdnL99mq7Q8oQQ", "hex" : "a914d9a625fb64a9faa4600548ae97e8efda9294dd0a87" } ],

Answer 1

As can be read in BOLT 11 of the lightning protocol spec

the f field allows on-chain fallback; however, this may not make sense for tiny or time-sensitive payments

it is one of the fields encoded into a BOLT 11 invoice

f (9): data_length variable, depending on version. Fallback on-chain address: for Bitcoin, this starts with a 5-bit version and contains a witness program or P2PKH or P2SH address.

In case a payment can't be routed it is possible to pay this invoice with an onchain bitcoin transaction instead of opening a new channel. This obviously can't be done for sub dust payments and even for tiny payments above the dust limit this might not be economically viable.